Mbufs leak since r321253

Hello,

While running some port builds over an NFS-mounted /usr/ports on my 
aarch64 VM after r321253, I've noticed that sooner or later I'm getting 
an error saying "kernel: [zone: mbuf_cluster] kern.ipc.nmbclusters limit 
reached". I've tried increasing the sysctl, but after some time I always 
got the error.

I've checked the mbuf counter with ``netstat -m'' periodically during 
the build, and noticed that the mbufs in use and mbuf clusters in use 
are continuously increasing, I've never seen a decrease:

279943/392/280335 mbufs in use (current/cache/total)
130819/253/131072/131072 mbuf clusters in use (current/cache/total/max)

According to 
https://svnweb.freebsd.org/base/head/?view=log&pathrev=321253, the 
previous commit to HEAD was r321248. I've checked it on that build as 
well, and I couldn't reproduce the issue, mbuf counters were stable.

After getting the kernel error message, the builds got stuck, some 
processes become unresponsive, and the only way to I was aware to clear 
the mbufs was to reboot the VM.

So far what I know, is building over NFS triggers the leak. However, I 
haven't tested it with other network activities, might also do the trick.

Could you please take a look into it?

If any more information is needed, please let me know, I still have my 
test VMs at hand, I can do a few tests.

Best regards,
Gergely