test: file_locks -- Use device from mountinfo for BTRFS

BTRFS uses own device numbering scheme so lets do a trick
close one to what we have in CRIU itself -- mangle
device to match mountinfo output.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>