libmount: fix mnt_table_is_fs_mounted() for rbind

libmount: fix mnt_table_is_fs_mounted() for rbind

There is no difference between "bind" and "rbind" if we want to FS
root to search for the FS in mountinfo file.

fstab:

 /dev/sdc1       /mnt/foo        xfs     defaults        0 0
 /mnt/foo        /mnt/test       none    rw,rbind        0 0

use -a more than once:

  mount -a
  mount -a

/proc/mounts (the current result):

 /dev/sdc1 /mnt/foo xfs rw,relatime,attr2,inode64,noquota 0 0
 /dev/sdc1 /mnt/test xfs rw,relatime,attr2,inode64,noquota 0 0
 /dev/sdc1 /mnt/test xfs rw,relatime,attr2,inode64,noquota 0 0
 /dev/sdc1 /mnt/foo xfs rw,relatime,attr2,inode64,noquota 0 0

expected (fixed version) result:

 /dev/sdc1 /mnt/foo xfs rw,relatime,attr2,inode64,noquota 0 0
 /dev/sdc1 /mnt/test xfs rw,relatime,attr2,inode64,noquota 0 0

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1528959
Signed-off-by: Karel Zak <kzak@redhat.com>