fscrypt: constify struct fscrypt_hkdf parameter to fscrypt_hkdf_expand()

Constify the struct fscrypt_hkdf parameter to fscrypt_hkdf_expand().
This makes it clearer that struct fscrypt_hkdf contains the key only,
not any per-request state.

Link: https://lore.kernel.org/r/20191209204054.227736-1-ebiggers@kernel.org


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