replace 'fill_pattern' with 'cpy_pattern' from 'lib/pattern.c'

replace 'fill_pattern' with 'cpy_pattern' from 'lib/pattern.c'

All pattern helpers are now located inside 'lib/pattern.c',
and 'cpy_pattern' should be sligtly faster, since it does
copy doubling the size on every iteration.

Signed-off-by: Roman Pen <r.peniaev@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>