Don't send the available space as is in the FSSTAT call.  Under FreeBSD, we can have a negative available space value, but the corresponding fields in the NFS protocol are unsigned.  So trnucate the value to 0 if it's negative, so that the client doesn't receive absurdly high values.
Don't send the available space as is in the FSSTAT call.  Under
FreeBSD, we can have a negative available space value, but the
corresponding fields in the NFS protocol are unsigned.  So
trnucate the value to 0 if it's negative, so that the client
doesn't receive absurdly high values.

Tested by:	cognet

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase