BACKPORT: loop: Add LOOP_SET_BLOCK_SIZE in compat ioctl

This change adds LOOP_SET_BLOCK_SIZE as one of the supported ioctls
in lo_compat_ioctl. It only takes an unsigned long argument, and
in practice a 32-bit value works fine.

Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 9fea4b395260175de4016b42982f45a3e6e03d0b)
[adelva: trivially backported around another backport conflict]
Bug: 150622092
Change-Id: I98e14d3ab60ca638c3aebcea942e2207c738af95
Signed-off-by: Alistair Delva <adelva@google.com>