fscrypt: don't check for ENOKEY from fscrypt_get_encryption_info()

fscrypt_get_encryption_info() returns 0 if the encryption key is
unavailable; it never returns ENOKEY.  So remove checks for ENOKEY.

Link: https://lore.kernel.org/r/20191209212348.243331-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>