fstrim: de-duplicate by mount source too

fstrim: de-duplicate by mount source too

Now fstrim de-duplicates by target (mountpoint). This patch adds
de-duplication according to mount source (device) to avoid bind mounts
or devices mounted more than once. Note that the patch also check FS
root, the different FS roots of the same multi-root FS (e.g. btrfs)
maybe mounted on different places.

  # mount --bind /home/wine /mnt/test

old version:

  # fstrim -av
  /mnt/test: 0 B (0 bytes) trimmed            <---
  /home/wine: 0 B (0 bytes) trimmed           <---
  /boot: 0 B (0 bytes) trimmed
  /home: 0 B (0 bytes) trimmed
  /: 0 B (0 bytes) trimmed

new version:

  # fstrim -av
  /mnt/test: 0 B (0 bytes) trimmed            <---
  /boot: 0 B (0 bytes) trimmed
  /home: 171.8 MiB (180113408 bytes) trimmed
  /: 0 B (0 bytes) trimmed

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1162213
Signed-off-by: Karel Zak <kzak@redhat.com>