xfs_repair: fix sizing of the incore rt space usage map calculation

xfs_repair: fix sizing of the incore rt space usage map calculation

If someone creates a realtime volume exactly *one* extent in length, the
sizing calculation for the incore rt space usage bitmap will be zero
because the integer division here rounds down.  Use howmany() to round
up.  Note that there can't be that many single-extent rt volumes since
repair will corrupt them into zero-extent rt volumes, and we haven't
gotten any reports.

Found by running xfs/530 after fixing xfs_repair to check the rt bitmap.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>