Inodes between subvolumes on a btrfs file system can have the same i_ino.
This messes with hard link detection during our file scan. To fix it, detect
when a file is on btrfs, and query the subvolume id. The filerec tree is
modified to sort on inum, then subvolid and thus we can detect duplicates.

For files not on btrfs, we store st_dev in the same location.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Merge branch 'subvol-checking'