[GFS2] Fix calculation of demote state

If a glock is in the exclusive state and a request for demote to
deferred has been received, then further requests for demote to
shared are being ignored. This patch fixes that by ensuring that
we demote to unlocked in that case.

Signed-off-by: Josef Whiter <jwhiter@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>