mptcp: use sha256() instead of open coding

Now that there's a function that calculates the SHA-256 digest of a
buffer in one step, use it instead of sha256_init() + sha256_update() +
sha256_final().

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Cc: mptcp@lists.01.org
Cc: Mat Martineau <mathew.j.martineau@linux.intel.com>
Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>