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