findmnt: improve -Q to output tree

findmnt: improve -Q to output tree

$ findmnt -Q 'FSTYPE=="ext4"'
TARGET         SOURCE                  FSTYPE OPTIONS
/              /dev/sda4               ext4   rw,relatime
├─/var         /dev/sda3               ext4   rw,relatime
│ └─/var/cache /dev/sda5               ext4   rw,relatime
├─/mnt/virt    /dev/sdb1               ext4   rw,relatime
└─/boot        /dev/sda2               ext4   rw,relatime

Addresses: https://github.com/util-linux/util-linux/pull/3090
Signed-off-by: Karel Zak <kzak@redhat.com>