test: Add test for private subdir

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Don't allocate fs_mount->rdst path

Full path to bind mount is "$root/$destination", we
used to calculate this one on ct spawn and save on
the fs_mount structure.

The problem is that bind mount is created by spawned
task and destroyed (sometimes) by master process, and
the latter doesn't know about changes made by former.

Generate full bindmount path always.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
test: Add test for bind mounts (non NEWNS case)

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