xfs: fix a comment in xfs_log_reserve

Fix the comment in xfs_log_reserve to avoid confusing.

Signed-of-by: Huang Chong <huang.chong@zte.com.cn>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>