make non-exchanging __d_move() copy ->d_parent rather than swap them

make non-exchanging __d_move() copy ->d_parent rather than swap them

Currently d_move(from, to) does the following:
	* name/parent of from <- old name/parent of to, from hashed there
	* to is unhashed
	* name of to is preserved
	* if from used to be detached, to gets detached
	* if from used to be attached, parent of to <- old parent of from.

That's both user-visibly bogus and complicates reasoning a lot.
Much saner semantics would be
	* name/parent of from <- name/parent of to, from hashed there.
	* to is unhashed
	* name/parent of to is unchanged.

The price, of course, is that old parent of from might lose a reference.
However,
	* all potentially cross-directory callers of d_move() have both
parents pinned directly; typically, dentries themselves are grabbed
only after we have grabbed and locked both parents.  IOW, the decrement
of old parent's refcount in case of d_move() won't reach zero.
	* __d_move() from d_splice_alias() is done to detached alias.
No refcount decrements in that case
	* __d_move() from __d_unalias() *can* get the refcount to zero.
So let's grab a reference to alias' old parent before calling __d_unalias()
and dput() it after we'd dropped rename_lock.

That does make d_splice_alias() potentially blocking.  However, it has
no callers in non-sleepable contexts (and the case where we'd grown
that dget/dput pair is _very_ rare, so performance is not an issue).

Another thing that needs adjustment is unlocking in the end of __d_move();
folded it in.  And cleaned the remnants of bogus ordering from the
"lock them in the beginning" counterpart - it's never been right and
now (well, for 7 years now) we have that thing always serialized on
rename_lock anyway.

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