Add a check for enforce_statfs != 0 to prison_check_nfsd()

[rmacklem]

Summary

Since mountd(8) will not be able to do exports
when running in a vnet prison if enforce_statfs is
set to 0, add a check for this to prison_check_nfsd().

Discussed over on D38144.

Test Plan

Tested in a system patched to allow mountd to
run in vnet prisons with enforce_statfs set to 0.

[jamie]

Sure, this one's no biggie.

[markj]

It looks like one can change enforce_statfs in an existing jail, so this doesn't seem quite sufficient? It might be useful to allow changing enforce_statfs after mountd starts though, so perhaps this is fine?

It'd be worth documenting this in jail(8) IMHO.