9p: fix ->rename_sem exclusion

9p wants to be able to build a path from given dentry to fs root and keep
it valid over a blocking operation.

->s_vfs_rename_mutex would be a natural candidate, but there are places
where we need that and where we have no way to tell if ->s_vfs_rename_mutex
is already held deeper in callchain.  Moreover, it's only held for
cross-directory renames; name changes within the same directory happen
without it.

Solution:
	* have d_move() done in ->rename() rather than in its caller
	* maintain a 9p-private rwsem (per-filesystem)
	* hold it exclusive over the relevant part of ->rename()
	* hold it shared over the places where we want the path.

That almost works.  FS_RENAME_DOES_D_MOVE is enough to put all d_move()
and d_exchange() calls under filesystem's control.  However, there's
also __d_unalias(), which isn't covered by any of that.

If ->lookup() hits a directory inode with preexisting dentry elsewhere
(due to e.g. rename done on server behind our back), d_splice_alias()
called by ->lookup() will move/rename that alias.

Add a couple of optional methods, so that __d_unalias() would do
	if alias->d_op->d_unalias_trylock != NULL
		if (!alias->d_op->d_unalias_trylock(alias))
			fail (resulting in -ESTALE from lookup)
	__d_move(...)
	if alias->d_op->d_unalias_unlock != NULL
		alias->d_unalias_unlock(alias)
where it currently does __d_move().  9p instances do down_write_trylock()
and up_write() of ->rename_mutex.

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