MFC: r221517

MFC: r221517
Change the new NFS server so that it returns 0 when the f_bavail
or f_ffree fields of "struct statfs" are negative, since the
values that go on the wire are unsigned and will appear to be
very large positive values otherwise. This makes the handling
of a negative f_bavail compatible with the old/regular NFS server.

git-svn-id: svn://svn.freebsd.org/base/stable/8@222109 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f