xfs: Add a comment in xfs_log_sb()

Add a comment explaining why the sb_frextents are updated outside the
if (xfs_has_lazycount(mp) check even though it is a lazycounter.
RT groups are supported only in v5 filesystems which always have
lazycounter enabled - so putting it inside the if(xfs_has_lazycount(mp)
check is redundant.

Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Nirjhar-Roy-0211

fs/xfs/libxfs/xfs_sb.c

@@ -1347,6 +1347,9 @@ xfs_log_sb(
 	 * feature was introduced.  This counter can go negative due to the way
 	 * we handle nearly-lockless reservations, so we must use the _positive
 	 * variant here to avoid writing out nonsense frextents.
+	 *
+	 * RT groups are only supported on v5 file systems, which always
+	 * have lazy SB counters.
 	 */
 	if (xfs_has_rtgroups(mp) && !xfs_has_zoned(mp)) {
 		mp->m_sb.sb_frextents =