mptcp: use HMAC-SHA256 library instead of open-coded HMAC

mptcp: use HMAC-SHA256 library instead of open-coded HMAC

Now that there are easy-to-use HMAC-SHA256 library functions, use these
in net/mptcp/crypto.c instead of open-coding the HMAC algorithm.

Remove the WARN_ON_ONCE() for messages longer than SHA256_DIGEST_SIZE.
The new implementation handles all message lengths correctly.

The mptcp-crypto KUnit test still passes after this change.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250902-net-next-mptcp-misc-feat-6-18-v2-1-fa02bb3188b1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>