gfs2: Fix refcount leak in gfs2_glock_poke

In gfs2_glock_poke, make sure gfs2_holder_uninit is called on the local
glock holder.  Without that, we're leaking a glock and a pid reference.

Fixes: 9e8990de

 ("gfs2: Smarter iopen glock waiting")
Cc: stable@vger.kernel.org # v5.8+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>