gfs2: Split the two kinds of glock "delete" work

Function delete_work_func() is used for two purposes:

 * to immediately try to evict the glock's inode, and

 * to verify after a little while that the inode has been deleted as
   expected, and didn't just get skipped.

These two operations are not separated very well, so introduce two new
glock flags to improved that.  Split gfs2_queue_delete_work() into
gfs2_queue_try_to_evict and gfs2_queue_verify_evict().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>