Sanity check COMMIT arguments by ensuring that (start)+(length) < 2^64.
The check is done in a way which is free of signedness pathologies in
all cases.

This change was inspired by pynfs, Peter Astrand's regression testsuite
for NFSv4 servers.  The change is necessary for all of the COMMIT tests
to pass.  However, it's a little open to debate whether the change is
really needed.  I'm curious to hear the opinions of other developers.