ceph, rbd: delete unnecessary checks before two function calls

The functions ceph_put_snap_context() and iput() test whether their
argument is NULL and then return immediately. Thus the test around the
call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[idryomov@redhat.com: squashed rbd.c hunk, changelog]
Signed-off-by: Ilya Dryomov <idryomov@redhat.com>