stat: Pass namespace into phys_stat_dev_match, not mnt tree

This makes the API simpler.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
stat: Pass namespace into phys_stat_resolve_dev, not mnt tree

This makes the API simpler.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mnt: Extend comment about how mntinfo->mountpoint path looks like

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