rbd: handle DISCARD and WRITE_ZEROES separately

With discard_zeroes_data gone in commit 48920ff2

 ("block: remove
the discard_zeroes_data flag"), continuing to provide this guarantee is
pointless: applications can't query it and discards can only be used
for deallocating.

Add OBJ_OP_ZEROOUT and move the existing logic under it.  As the first
step to divorcing OBJ_OP_DISCARD, stop worrying about copyups but keep
special casing whole-object layered discards.

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