locks: fix NULL-deref in generic_delete_lease

locks: fix NULL-deref in generic_delete_lease

commit ac4ac6afc8f81d6dec5db11a140163ec4184bc44
  locks: generic_delete_lease doesn't need a file_lock at all

moves the call to fl->fl_lmops->lm_change() to a place in the
code where fl might be a non-lease lock.
When that happens, fl_lmops is NULL and an Oops ensures.

So add an extra test to restore correct functioning.

Reported-by: Linda Walsh <suse@tlinx.org>
Link: https://bugzilla.suse.com/show_bug.cgi?id=912569
Cc: stable@vger.kernel.org (v3.18)
Fixes: ac4ac6afc8f81d6dec5db11a140163ec4184bc44
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>