move_mount(2): take sanity checks in 'beneath' case into do_lock_mount()

move_mount(2): take sanity checks in 'beneath' case into do_lock_mount()

We want to mount beneath the given location.  For that operation to
make sense, location must be the root of some mount that has something
under it.  Currently we let it proceed if those requirements are not met,
with rather meaningless results, and have that bogosity caught further
down the road; let's fail early instead - do_lock_mount() doesn't make
sense unless those conditions hold, and checking them there makes
things simpler.

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