xfs: lock the AIL before removing the buffer item

Regression introduced by commit 46f9d2e ("xfs: aborted buf items can
be in the AIL") which fails to lock the AIL before removing the
item. Spinlock debugging throws a warning about this.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>