audit: Don't pass pid or uid to audit_log_common_recv_msg

The only place we use the uid and the pid that we calculate in
audit_receive_msg is in audit_log_common_recv_msg so move the
calculation of these values into the audit_log_common_recv_msg.

Simplify the calcuation of the current pid and uid by
reading them from current instead of reading them from
NETLINK_CREDS.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>