xfs: reset rootdir extent size hint after growfsrt

If growfsrt is run on a filesystem that doesn't have a rt volume, it's
possible to change the rt extent size.  If the root directory was
previously set up with an inherited extent size hint and rtinherit, it's
possible that the hint is no longer a multiple of the rt extent size.
Although the verifiers don't complain about this, xfs_repair will, so if
we detect this situation, log the root directory to clean it up.  This
is still racy, but it's better than nothing.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>