gfs2: Only defer deletes when we have an iopen glock

The mechanism to defer deleting unlinked inodes is tied to
delete_work_func(), which is tied to iopen glocks.  When we don't have
an iopen glock, we must carry out deletes immediately instead.

Fixes a NULL pointer dereference in gfs2_evict_inode().

Fixes: 8c21c2c7 ("gfs2: Call gfs2_queue_verify_delete from gfs2_evict_inode")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>