xfs: memory barrier before wake_up_bit()

In xfs_ifunlock() there is a call to wake_up_bit() after clearing
the flush lock on the xfs inode.  This is not guaranteed to be safe,
as noted in the comments above wake_up_bit() beginning with:

    In order for this to function properly, as it uses
    waitqueue_active() internally, some kind of memory
    barrier must be done prior to calling this.


Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Ben Myers <bpm@sgi.com>