xfs: Add a comment in xfs_log_sb()

xfs: Add a comment in xfs_log_sb()

Source kernel commit: ac1d977096a17d56c55bd7f90be48e81ac4cec3f

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>