gfs2: Save ip from gfs2_glock_nq_init

Before this patch, when a glock was locked by function gfs2_glock_nq_init,
it initialized the holder gh_ip (return address) as gfs2_glock_nq_init.
That made it extremely difficult to track down problems because many
functions call gfs2_glock_nq_init. This patch changes the function so
that it saves gh_ip from the caller of gfs2_glock_nq_init, which makes
it easy to backtrack which holder took the lock.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>