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>