bootp is broken in 4.6.0 and git

Hi.

I've found that bootp doesn't work in 4.6.0. I looked at current git and found the commit

commit c9f314f6e315a5518432761fea864196a290f799
Author: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
Date:   Thu Jun 17 18:01:32 2021 +0900

    Fix "DHCP broken in libslirp v4.6.0"

I built master at 146ff910 and no, it doesn't work. I then did a git bisect.

f13cad45b25d92760bb0ad67bec0300a4d7d5275 is the first bad commit
commit f13cad45b25d92760bb0ad67bec0300a4d7d5275
Author: Marc-André Lureau <marcandre.lureau@redhat.com>
Date:   Fri Jun 4 19:25:28 2021 +0400

    bootp: limit vendor-specific area to input packet memory buffer

Unfortunately it doesn't cleanly revert from master, so I can't test master with this patch reverted.

Please let me know if I can do more testing.