BTRFS returns subvolume dev-id instead of superblock dev-id,
so we need to know which mounts are btrfs.

The mi->fstype->name is "unsuppoerted" here, because the fstype->code
is saved in an image

{
.name = "unsupported",
.code = FSTYPE__UNSUPPORTED,
},
{
.name = "btrfs",
.code = FSTYPE__UNSUPPORTED,
}

An a second reason is that pocesses can be migrated from smth to btrfs.
This all can happen _only_ for the root mount and for bind mounts of
the root mount...
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>