BACKPORT: block/loop: set hw_sectors

Loop can handle any size of request. Limiting it to 255 sectors just
burns the CPU for bio split and request merge for underlayer disk and
also cause bad fs block allocation in directio mode.

Bug: 117823094
Change-Id: Ic4957181433c5a0d15f4cfdbf69dc5558d6dc5bd
Reviewed-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 54bb0ade6627a183c211345761ec46e4bf0048fe)
Signed-off-by: Martijn Coenen <maco@android.com>