Serious overflow/signedness issue in NFS server

[Garrett Wollman]

Yesterday I upgraded my production NFS servers to 10.1 from 9.3.
Very quickly, my users ran into the kernel RPC's buffer space
throttling mechanism.  Besides having a stupidly low and arbitrary
hard-coded limit, this code also has overflow bugs which which were
exposed by the switch to clang as the system compiler.

Please have a look at <https://reviews.freebsd.org/D2165> for what I
think is going to be the fix, and if you have a FreeBSD phabricator
account, please sign on as a reviewer.

I'm sure there are other lingering overflow bugs in this code.

-GAWollman