Many places in the gfs2 code queued and dequeued the freeze glock.
Almost all of them acquire it in SHARED mode, and need to specify the
same LM_FLAG_NOEXP and GL_EXACT flags.

This patch adds common helper functions gfs2_freeze_lock and gfs2_freeze_unlock
to make the code more readable, and to prepare for the next patch.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>