selinux: add netlink nlmsg_type audit message

Add a new audit message type to capture nlmsg-related information. This
is similar to LSM_AUDIT_DATA_IOCTL_OP which was added for the other
SELinux extended permission (ioctl).

Adding a new type is preferred to adding to the existing
lsm_network_audit structure which contains irrelevant information for
the netlink sockets (i.e., dport, sport).

Signed-off-by: Thiébaud Weksteen <tweek@google.com>
[PM: change "nlnk-msgtype" to "nl-msgtype" as discussed]
Signed-off-by: Paul Moore <paul@paul-moore.com>