netfs: Support decryption on ubuffered/DIO read

Support unbuffered and direct I/O reads from an encrypted file.  This may
require making a larger read than is required into a bounce buffer and
copying out the required bits.  We don't decrypt in-place in the user
buffer lest userspace interfere and muck up the decryption.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org