sctp: Remove unused payload from sctp_idatahdr

Remove the unused payload array from the struct sctp_idatahdr.

Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20250223204505.2499-3-thorsten.blum@linux.dev


Signed-off-by: Paolo Abeni <pabeni@redhat.com>