crypto: xts - Drop use of auxiliary buffer

Since commit acb9b159c784 ("crypto: gf128mul - define gf128mul_x_* in
gf128mul.h"), the gf128mul_x_*() functions are very fast and therefore
caching the computed XTS tweaks has only negligible advantage over
computing them twice.

In fact, since the current caching implementation limits the size of
the calls to the child ecb(...) algorithm to PAGE_SIZE (usually 4096 B),
it is often actually slower than the simple recomputing implementation.

This patch simplifies the XTS template to recompute the XTS tweaks from
scratch in the second pass and thus also removes the need to allocate a
dynamic buffer using kmalloc().

As discussed at [1], the use of kmalloc causes deadlocks with dm-crypt.

PERFORMANCE RESULTS
I measured time to encrypt/decrypt a memory buffer of varying sizes with
xts(ecb-aes-aesni) using a tool I wrote ([2]) and the results suggest
that after this patch the performance is either better or comparable for
both small and large buffers. Note that there is a lot of noise in the
measurements, but the overall difference is easy to see.

Old code:
       ALGORITHM KEY (b)        DATA (B)   TIME ENC (ns)   TIME DEC (ns)
        xts(aes)     256              64             331             328
        xts(aes)     384              64             332             333
        xts(aes)     512              64             338             348
        xts(aes)     256             512             889             920
        xts(aes)     384             512            1019             993
        xts(aes)     512             512            1032             990
        xts(aes)     256            4096            2152            2292
        xts(aes)     384            4096            2453            2597
        xts(aes)     512            4096            3041            2641
        xts(aes)     256           16384            9443            8027
        xts(aes)     384           16384            8536            8925
        xts(aes)     512           16384            9232            9417
        xts(aes)     256           32768           16383           14897
        xts(aes)     384           32768           17527           16102
        xts(aes)     512           32768           18483           17322

New code:
       ALGORITHM KEY (b)        DATA (B)   TIME ENC (ns)   TIME DEC (ns)
        xts(aes)     256              64             328             324
        xts(aes)     384              64             324             319
        xts(aes)     512              64             320             322
        xts(aes)     256             512             476             473
        xts(aes)     384             512             509             492
        xts(aes)     512             512             531             514
        xts(aes)     256            4096            2132            1829
        xts(aes)     384            4096            2357            2055
        xts(aes)     512            4096            2178            2027
        xts(aes)     256           16384            6920            6983
        xts(aes)     384           16384            8597            7505
        xts(aes)     512           16384            7841            8164
        xts(aes)     256           32768           13468           12307
        xts(aes)     384           32768           14808           13402
        xts(aes)     512           32768           15753           14636

[1] https://lkml.org/lkml/2018/8/23/1315
[2] https://gitlab.com/omos/linux-crypto-bench

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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