qa/workunits/rbd: fixups for the new krbd discard behavior

qa/workunits/rbd: fixups for the new krbd discard behavior

With discard_granularity set to alloc_size, we no longer get object
size alignment from blk_bio_discard_split().

This assumption is pretty deeply ingrained in krbd_data_pool.sh, so
make it explicit.  For krbd_fallocate.sh, just fix the expectation.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit f3f757aaa39c73615749926f6503c2e83ca7f0d1)

Conflicts:
	qa/workunits/rbd/krbd_fallocate.sh
- luminous does not have a7d89c07946f89ad28987d65d9ba71c464c32431
  so we keep the function name "assert_deallocated_unaligned"