gfs2: Fix request cancelation bug

In finish_xmote(), when a locking request is canceled, the corresponding
holder is moved to the tail of the holders list instead of being
dequeued immediately.  When there is only a single holder, the canceled
locking request is then immediately repeated.  This makes no sense; it
looks like another remnant of LM_FLAG_PRIORITY support.

Instead, dequeue canceled holders and proceed with the next holder in
finish_xmote().  We can then easily detect in gfs2_glock_dq() when a
holder has been canceled.

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