GFS2: Always use iopen glock for gl_deletes

Before this patch, when function try_rgrp_unlink queued a glock for
delete_work to reclaim the space, it used the inode glock to do so.
That's different from the iopen callback which uses the iopen glock
for the same purpose. We should be consistent and always use the
iopen glock. This may also save us reference counting problems with
the inode glock, since clear_glock does an extra glock_put() for the
inode glock.

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