Optimize pattern verify

Optimize pattern verify

Similar to the patch last week, this optimizes the pattern verify
operation to use optimized library calls like memcmp(), and only fall
back to byte-by-byte if there is a miscompare to locate it.

This uses the same premise that the pattern is repeated as many times
as possible to do large compares in a single call.  For single byte
pattern, the setup fills the pattern space, and verify assumes it is
full.

Tested by running a script which created an 8k file with 4k bs and,
one byte at a time, tried corrupting the pattern and running a read
pass and verified it still found miscompares across the whole range of
the pattern.  This was done with a pattern length of 1 and 3 bytes.

In performance tests, this was about 8 times more efficient on
verifies than without this patch.

Signed-off-by: Jens Axboe <axboe@kernel.dk>