proc: Fix anon shmem mapping detection

Linux kernel emulates anon-shared mappings by mapping internal
tmpfs file in. We try to detect this by checking that the file
under map is such, but do it with error -- major == 0 check is
wrong, as regular tmpfs file can be such as well as btrfs or
ecryptfs can screw things up.

The only working way of doing this is to get the dev_t of this
internal tmpfs mount.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>