netlink: fix compilation after memory mapped patches

Depending of the kernel configuration (CONFIG_UIDGID_STRICT_TYPE_CHECKS), we can
get the following errors:

net/netlink/af_netlink.c: In function ‘netlink_queue_mmaped_skb’:
net/netlink/af_netlink.c:663:14: error: incompatible types when assigning to type ‘__u32’ from type ‘kuid_t’
net/netlink/af_netlink.c:664:14: error: incompatible types when assigning to type ‘__u32’ from type ‘kgid_t’
net/netlink/af_netlink.c: In function ‘netlink_ring_set_copied’:
net/netlink/af_netlink.c:693:14: error: incompatible types when assigning to type ‘__u32’ from type ‘kuid_t’
net/netlink/af_netlink.c:694:14: error: incompatible types when assigning to type ‘__u32’ from type ‘kgid_t’

We must use the helpers to get the uid and gid, and also take care of user_ns.

Fix suggested by Eric W. Biederman <ebiederm@xmission.com>.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>