dlm: fix user space lkb refcounting

This patch fixes to check on the right return value if it was the last
callback. The rv variable got overwritten by the return of
copy_result_to_user(). Fixing it by introducing a second variable for
the return value and don't let rv being overwritten.

Cc: stable@vger.kernel.org
Fixes: 61bed0ba ("fs: dlm: use a non-static queue for callbacks")
Reported-by: Valentin Vidić <vvidic@valentin-vidic.from.hr>
Closes: https://lore.kernel.org/gfs2/Ze4qSvzGJDt5yxC3@valentin-vidic.from.hr


Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>