net: fix SOL_NETLINK NETLINK_LIST_MEMBERSHIPS decoding

net: fix SOL_NETLINK NETLINK_LIST_MEMBERSHIPS decoding

NETLINK_LIST_MEMBERSHIPS, unlike all other SOL_NETLINK options, requests
not just a single integer but an array of integers.  The kernel also
supports a zero optlen NETLINK_LIST_MEMBERSHIPS request.

* net.c (print_uint32): New function.
(print_getsockopt): Add ulen argument, rename len argument to rlen,
<SOL_NETLINK> Handle NETLINK_LIST_MEMBERSHIPS using print_array
and print_uint32.
(SYS_FUNC(getsockopt)): Pass ulen to print_getsockopt.
* tests/sockopt-sol_netlink.c (main): Check NETLINK_LIST_MEMBERSHIPS
decoding.