audit: check the length of userspace generated audit records

commit 763dafc5 upstream.

Commit 75612528 ("audit: always check the netlink payload length
in audit_receive_msg()") fixed a number of missing message length
checks, but forgot to check the length of userspace generated audit
records.  The good news is that you need CAP_AUDIT_WRITE to submit
userspace audit records, which is generally only given to trusted
processes, so the impact should be limited.

Cc: stable@vger.kernel.org
Fixes: 75612528 ("audit: always check the netlink payload length in audit_receive_msg()")
Reported-by:  <syzbot+49e69b4d71a420ceda3e@syzkaller.appspotmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>