tests: fix potential unaligned access in netlink_sock_diag.test

tests: fix potential unaligned access in netlink_sock_diag.test

Despite our efforts made to avoid unaligned access, gcc appears to be
generating instructions that might cause unaligned access when copying
structures to unaligned addresses.

Fix this issue by replacing copying of structures to unaligned addresses
with memcpy calls.

* tests/netlink_sock_diag.c (INIT_STRUCT): New macro.
(test_odd_family_req, test_odd_family_msg, test_unix_diag_req,
test_unix_diag_msg, test_netlink_diag_req, test_netlink_diag_msg,
test_packet_diag_req, test_packet_diag_msg, test_inet_diag_sockid,
test_inet_diag_req, test_inet_diag_req_v2, test_inet_diag_msg,
test_smc_diag_req, test_smc_diag_msg): Use it to initialize potentially
unaligned struct nlmsghdr.  Do not access nlh->nlmsg_len.