GFS2: Don't cache iopen glocks

This patch makes GFS2 immediately reclaim/delete all iopen glocks
as soon as they're dequeued. This allows deleters to get an
EXclusive lock on iopen so files are deleted properly instead of
being set as unlinked.

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