GFS2: Combine functions gfs2_glock_dq_wait and wait_on_demote

Function gfs2_glock_dq_wait called two-line function wait_on_demote,
so they were combined.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>