migrate: Remove references to Private2

These comments are now stale; rewrite them.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241002040111.1023018-7-willy@infradead.org


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>