src/fssum: skip subvolumes when building a sum

src/fssum: skip subvolumes when building a sum

With the snapshot/subvolume support added to fsstress I've been seeing
random failures with our send/receive related tests.  This is because
fssum is summing the path with the subvolumes for our test fs'es that
are generated by fsstress.  But with send/receive it skips subvolumes,
which makes the sums mismatch.  Fix this by skipping directories that do
not match our st_dev, which is how we differentiate subvolumes in btrfs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>