xfs: make use of XFS_SB_LOG_RES() at xfs_fs_log_dummy()

Make use of XFS_SB_LOG_RES() at xfs_fs_log_dummy().

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
CC: Dave Chinner <david@fromorbit.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>