do_lock_mount(): don't modify path.

Currently do_lock_mount() has the target path switched to whatever
might be overmounting it.  We _do_ want to have the parent
mount/mountpoint chosen on top of the overmounting pile; however,
the way it's done has unpleasant races - if umount propagation
removes the overmount while we'd been trying to set the environment
up, we might end up failing if our target path strays into that overmount
just before the overmount gets kicked out.

Users of do_lock_mount() do not need the target path changed - they
have all information in res->{parent,mp}; only one place (in
do_move_mount()) currently uses the resulting path->mnt, and that value
is trivial to reconstruct by the original value of path->mnt + chosen
parent mount.

Let's keep the target path unchanged; it avoids a bunch of subtle races
and it's not hard to do:
	do
		as mount_locked_reader
			find the prospective parent mount/mountpoint dentry
			grab references if it's not the original target
		lock the prospective mountpoint dentry
		take namespace_sem exclusive
		if prospective parent/mountpoint would be different now
			err = -EAGAIN
		else if location has been unmounted
			err = -ENOENT
		else if mountpoint dentry is not allowed to be mounted on
			err = -ENOENT
		else if beneath and the top of the pile was the absolute root
			err = -EINVAL
		else
			try to get struct mountpoint (by dentry), set
			err to 0 on success and -ENO{MEM,ENT} on failure
		if err != 0
			res->parent = ERR_PTR(err)
			drop locks
		else
			res->parent = prospective parent
		drop temporary references
	while err == -EAGAIN

A somewhat subtle part is that dropping temporary references is allowed.
Neither mounts nor dentries should be evicted by a thread that holds
namespace_sem.  On success we are dropping those references under
namespace_sem, so we need to be sure that these are not the last
references remaining.  However, on success we'd already verified (under
namespace_sem) that original target is still mounted and that mount
and dentry we are about to drop are still reachable from it via the
mount tree.  That guarantees that we are not about to drop the last
remaining references.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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