VFS/ovl/smb: introduce start_renaming_dentry()

Several callers perform a rename on a dentry they already have, and only
require lookup for the target name.  This includes smb/server and a few
different places in overlayfs.

start_renaming_dentry() performs the required lookup and takes the
required lock using lock_rename_child()

It is used in three places in overlayfs and in ksmbd_vfs_rename().

In the ksmbd case, the parent of the source is not important - the
source must be renamed from wherever it is.  So start_renaming_dentry()
allows rd->old_parent to be NULL and only checks it if it is non-NULL.
On success rd->old_parent will be the parent of old_dentry with an extra
reference taken.  Other start_renaming function also now take the extra
reference and end_renaming() now drops this reference as well.

ovl_lookup_temp(), ovl_parent_lock(), and ovl_parent_unlock() are
all removed as they are no longer needed.

OVL_TEMPNAME_SIZE and ovl_tempname() are now declared in overlayfs.h so
that ovl_check_rename_whiteout() can access them.

ovl_copy_up_workdir() now always cleans up on error.

Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-12-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

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