fscrypt: support decrypting data from large folios

Try to make the filesystem-level decryption functions in fs/crypto/
aware of large folios.  This includes making fscrypt_decrypt_bio()
support the case where the bio contains large folios, and making
fscrypt_decrypt_pagecache_blocks() take a folio instead of a page.

There's no way to actually test this with large folios yet, but I've
tested that this doesn't cause any regressions.

Note that this patch just handles *decryption*, not encryption which
will be a little more difficult.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20230127224202.355629-1-ebiggers@kernel.org

fs/buffer.c

@@ -331,8 +331,8 @@ static void decrypt_bh(struct work_struct *work)
 	struct buffer_head *bh = ctx->bh;
 	int err;
 
-	err = fscrypt_decrypt_pagecache_blocks(bh->b_page, bh->b_size,
-					       bh_offset(bh));
+	err = fscrypt_decrypt_pagecache_blocks(page_folio(bh->b_page),
+					       bh->b_size, bh_offset(bh));
 	if (err == 0 && need_fsverity(bh)) {
 		/*
 		 * We use different work queues for decryption and for verity