xfs: don't scan off the end of the rt volume in xfs_rtallocate_extent_block

xfs: don't scan off the end of the rt volume in xfs_rtallocate_extent_block

The loop conditional here is not quite correct because an rtbitmap block
can represent rtextents beyond the end of the rt volume.  There's no way
that it makes sense to scan for free space beyond EOFS, so don't do it.
This overrun has been present since v2.6.0.

Also fix the type of bestlen, which was incorrectly converted.

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