fs: Turn page_offset() into a wrapper around folio_pos()

This is far less efficient for the lagging filesystems which still
use page_offset(), but it removes an access to page->index.  It also
fixes a bug -- if any filesystem passed a tail page to page_offset(),
it would return garbage which might result in the filesystem choosing
to not writeback a dirty page.  There probably aren't any examples
of this, but I can't be certain.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250221203932.3588740-1-willy@infradead.org


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