rbd: round off and ignore discards that are too small

If, after rounding off, the discard request is smaller than alloc_size,
drop it on the floor in __rbd_img_fill_request().

Default alloc_size to 64k.  This should cover both HDD and SSD based
bluestore OSDs and somewhat improve things for filestore.  For OSDs on
filestore with filestore_punch_hole = false, alloc_size is best set to
object size in order to allow deletes and truncates and disallow zero
op.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>