ecryptfs: Convert lower_offset_for_page() to take a folio

Both callers have a folio, so pass it in and use folio->index instead of
page->index.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241025190822.1319162-10-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>