vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

vfs_rename_dir() doesn't properly account for filesystems with
FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it
unhashes the new_dentry prior to the rename() iop and rehashes it after,
but doesn't account for the possibility that rename() may have swapped
{old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it rehashes
new_dentry (now the old renamed-from name, which d_move() expected to go
away), such that a subsequent lookup will find it.  Currently all
FS_RENAME_DOES_D_MOVE filesystems compensate for this by failing in
d_revalidate.

The bug was introduced by: commit 4d468d3b7e02e68a45d7bdfd21bdee180f74a200
"[PATCH] Allow file systems to manually d_move() inside of ->rename()"

Fix by not rehashing the new dentry.  Rehashing used to be needed by
d_move() but isn't anymore.

Reported-by: Sage Weil <sage@newdream.net>
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>