xfs: do exact log space wakeups in xlog_ungrant_log_space

The only reason that xfs_log_space_wake had to do opportunistic wakeups
was that the old xfs_log_move_tail calling convention didn't allow for
exact wakeups when not updating the log tail LSN.  Since this issue has
been fixed we can do exact wakeups now.

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