sctp: Fix MAC comparison to be constant-time

To prevent timing attacks, MACs need to be compared in constant time.
Use the appropriate helper function for this.

Fixes: bbd0d598 ("[SCTP]: Implement the receive and verification of AUTH chunk")
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20250818205426.30222-3-ebiggers@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>