KEYS: encrypted: avoid encrypting/decrypting stack buffers

KEYS: encrypted: avoid encrypting/decrypting stack buffers

Since v4.9, the crypto API cannot (normally) be used to encrypt/decrypt
stack buffers because the stack may be virtually mapped.  Fix this for
the padding buffers in encrypted-keys by using ZERO_PAGE for the
encryption padding and by allocating a temporary heap buffer for the
decryption padding.

Tested with CONFIG_DEBUG_SG=y:
	keyctl new_session
	keyctl add user master "abcdefghijklmnop" @s
	keyid=$(keyctl add encrypted desc "new user:master 25" @s)
	datablob="$(keyctl pipe $keyid)"
	keyctl unlink $keyid
	keyid=$(keyctl add encrypted desc "load $datablob" @s)
	datablob2="$(keyctl pipe $keyid)"
	[ "$datablob" = "$datablob2" ] && echo "Success!"

Cc: Andy Lutomirski <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Mimi Zohar <[email protected]>
Cc: [email protected] # 4.9+
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: David Howells <[email protected]>
Signed-off-by: James Morris <[email protected]>