xfs: recompute growfsrtfree transaction reservation while growing rt volume

While playing with growfs to create a 20TB realtime section on a
filesystem that didn't previously have an rt section, I noticed that
growfs would occasionally shut down the log due to a transaction
reservation overflow.

xfs_calc_growrtfree_reservation uses the current size of the realtime
summary file (m_rsumsize) to compute the transaction reservation for a
growrtfree transaction.  The reservations are computed at mount time,
which means that m_rsumsize is zero when growfs starts "freeing" the new
realtime extents into the rt volume.  As a result, the transaction is
undersized and fails.

Fix this by recomputing the transaction reservations every time we
change m_rsumsize.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_rtalloc.c

@@ -1038,6 +1038,9 @@ xfs_growfs_rt(
 		nrsumblocks = xfs_rtsummary_blockcount(mp, nrsumlevels,
 				nsbp->sb_rbmblocks);
 		nmp->m_rsumsize = nrsumsize = XFS_FSB_TO_B(mp, nrsumblocks);
+		/* recompute growfsrt reservation from new rsumsize */
+		xfs_trans_resv_calc(nmp, &nmp->m_resv);
+
 		/*
 		 * Start a transaction, get the log reservation.
 		 */
@@ -1124,6 +1127,8 @@ xfs_growfs_rt(
 		 */
 		mp->m_rsumlevels = nrsumlevels;
 		mp->m_rsumsize = nrsumsize;
+		/* recompute growfsrt reservation from new rsumsize */
+		xfs_trans_resv_calc(mp, &mp->m_resv);
 
 		error = xfs_trans_commit(tp);
 		if (error)