aio: use cancellation list lazily

Cancelling kiocbs requires adding them to a per kioctx linked list,
which is one of the few things we need to take the kioctx lock for in
the fast path.  But most kiocbs can't be cancelled - so if we just do
this lazily, we can avoid quite a bit of locking overhead.

While we're at it, instead of using a flag bit switch to using ki_cancel
itself to indicate that a kiocb has been cancelled/completed.  This lets
us get rid of ki_flags entirely.

[akpm@linux-foundation.org: remove buggy BUG()]
Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Zach Brown <zab@redhat.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Asai Thambi S P <asamymuthupa@micron.com>
Cc: Selvan Mani <smani@micron.com>
Cc: Sam Bradshaw <sbradshaw@micron.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Reviewed-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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