audit: always check the netlink payload length in audit_receive_msg()

This patch ensures that we always check the netlink payload length
in audit_receive_msg() before we take any action on the payload
itself.

Cc: stable@vger.kernel.org
Reported-by:  <syzbot+399c44bf1f43b8747403@syzkaller.appspotmail.com>
Reported-by:  <syzbot+e4b12d8d202701f08b6d@syzkaller.appspotmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>