fs: update kernel-doc for vfs_rename()

Commit 9fe61450

 ("namei: introduce struct renamedata") introduces a
new struct for vfs_rename() and makes the vfs_rename() kernel-doc argument
description out of sync.

Move the description of arguments for vfs_rename() to a new kernel-doc for
the struct renamedata to make these descriptions checkable against the
actual implementation.

Link: https://lore.kernel.org/r/20210204180059.28360-3-lukas.bulwahn@gmail.com
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>