fscrypt: Change fscrypt_encrypt_pagecache_blocks() to take a folio

fscrypt: Change fscrypt_encrypt_pagecache_blocks() to take a folio

ext4 and ceph already have a folio to pass; f2fs needs to be properly
converted but this will do for now.  This removes a reference
to page->index and page->mapping as well as removing a call to
compound_head().

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250304170224.523141-1-willy@infradead.org
Acked-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>