do_move_mount(): use the parent mount returned by do_lock_mount()

do_move_mount(): use the parent mount returned by do_lock_mount()

After successful do_lock_mount() call, mp.parent is set to either
real_mount(path->mnt) (for !beneath case) or to ->mnt_parent of that
(for beneath).  p is set to real_mount(path->mnt) and after
several uses it's made equal to mp.parent.  All uses prior to that
care only about p->mnt_ns and since p->mnt_ns == parent->mnt_ns,
we might as well use mp.parent all along.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>