Optimize pattern filling by limiting small calls to memcpy

Optimize pattern filling by limiting small calls to memcpy

In looking at profiling the speed of fill_pattern(), it calls memcpy()
for the fill pattern repeatedly for multibyte patterns.  So for a 4
byte pattern with 8k IO, it calls memcpy() 2048 times.

Since there is already 512 bytes reserved for the pattern, I figured a
simple solution was to use it.  This patch replicates short patterns
so they can be more efficiently copied.  (Single byte patterns are
left alone since they can make use of the much more efficient memset()
call.)

The result is a 10x performance improvement on pattern filling. (With
this patch, it's still 3x slower than when it re-uses the already
filled pattern.)

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