librbd: align discard requests to default bluestore allocation size

librbd: align discard requests to default bluestore allocation size

This will match the new krbd behavior of aligning discards to the
default bluestore minimum allocation size. Requests that are too
small (or the misaligned head / tail of the request) will be
ignored since it will not result in freed space.

Fixes: http://tracker.ceph.com/issues/38146
Signed-off-by: Jason Dillaman <dillaman@redhat.com>