nfsd: Fix f_bavail and f_ffree for NFSv4 when negative

Since the NFS Space_available and Files_available are unsigned,
the NFSv3 server sets them to 0 when negative, so that they
do not appear to be large positive values for non-FreeBSD clients.
This patch fixes the NFSv4 server to do the same.

Found during a recent IEFT NFSv4 working group testing event.

MFC after: 2 weeks