xfs: fix an integer overflow error in xfs_growfs_rt

During a realtime grow operation, we run a single transaction for each
rt bitmap block added to the filesystem.  This means that each step has
to be careful to increase sb_rblocks appropriately.

Fix the integer overflow error in this calculation that can happen when
the extent size is very large.  Found by running growfs to add a rt
volume to a filesystem formatted with a 1g rt extent size.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_rtalloc.c

@@ -1021,7 +1021,8 @@ xfs_growfs_rt(
 		     ((sbp->sb_rextents & ((1 << mp->m_blkbit_log) - 1)) != 0);
 	     bmbno < nrbmblocks;
 	     bmbno++) {
-		xfs_trans_t	*tp;
+		struct xfs_trans	*tp;
+		xfs_rfsblock_t		nrblocks_step;
 
 		*nmp = *mp;
 		nsbp = &nmp->m_sb;
@@ -1030,10 +1031,9 @@ xfs_growfs_rt(
 		 */
 		nsbp->sb_rextsize = in->extsize;
 		nsbp->sb_rbmblocks = bmbno + 1;
-		nsbp->sb_rblocks =
-			XFS_RTMIN(nrblocks,
-				  nsbp->sb_rbmblocks * NBBY *
-				  nsbp->sb_blocksize * nsbp->sb_rextsize);
+		nrblocks_step = (bmbno + 1) * NBBY * nsbp->sb_blocksize *
+				nsbp->sb_rextsize;
+		nsbp->sb_rblocks = min(nrblocks, nrblocks_step);
 		nsbp->sb_rextents = nsbp->sb_rblocks;
 		do_div(nsbp->sb_rextents, nsbp->sb_rextsize);
 		ASSERT(nsbp->sb_rextents != 0);