xfs_repair: coordinate parallel updates to the rt bitmap

xfs_repair: coordinate parallel updates to the rt bitmap

Actually take the rt lock before updating the bitmap from multiple
threads.  This fixes an infrequent corruption problem when running
generic/013 and rtinherit=1 is set on the root dir.

Fixes: 2556c98bd9e6 ("Perform true sequential bulk read prefetching in xfs_repair Merge of master-melb:xfs-cmds:29147a by kenmcd.")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>