net: sctp: sctp_auth_make_key_vector: remove duplicate ntohs calls

Instead of calling 3 times ntohs(random->param_hdr.length), 2 times
ntohs(hmacs->param_hdr.length), and 3 times ntohs(chunks->param_hdr.length)
within the same function, we only call each once and store it in a
variable.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>