BTRFS returns subvolume dev-id instead of superblock dev-id,
in such case return device obtained from mountinfo (ie subvolume0).

v2: fix up devices only for btrfs files.
v3: use phys_stat_dev_match instead of phys_stat_resolve_dev
v4: fix cosmetic whims

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>