If you are trying to backup your root / file system which is mounted at / you should use / instead of the partition device.
/usr/sbin/xfsdump -v verbose -M 1 -L 1 -l 0 -f /lin-bak/2017-08-1121-39-07md126p2.XfsDump /
The XFS file system you are trying to back up is mounted at / so just use //
xfsdump - XFS filesystem incremental dump utility
xfsdump [ options ] -f dest [ -f dest ... ] filesystem
here filesystem is the top level directory in this case /.
xfsdump works on XFS file system contents not on the partitions that contain them.
You don't need to know the partition on which a file system is mounted. All of your commands with df and grep were totally unnecessary.