s390/bpf: Make sure JIT passes do not increase code size

The upcoming s390 branch length extension patches rely on "passes do
not increase code size" property in order to consistently choose between
short and long branches. Currently this property does not hold between
the first and the second passes for register save/restore sequences, as
well as various code fragments that depend on SEEN_* flags.

Generate the code during the first pass conservatively: assume register
save/restore sequences have the maximum possible length, and that all
SEEN_* flags are set.

Also refuse to JIT if this happens anyway (e.g. due to a bug), as this
might lead to verifier bypass once long branches are introduced.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191114151820.53222-1-iii@linux.ibm.com

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