svcrpc: implement O_NONBLOCK behavior for use-gss-proxy

Somebody noticed LTP was complaining about O_NONBLOCK opens of
/proc/net/rpc/use-gss-proxy succeeding and then a following read
hanging.

I'm not convinced LTP really has any business opening random proc files
and expecting them to behave a certain way.  Maybe this isn't really a
bug.

But in any case the O_NONBLOCK behavior could be useful for someone that
wants to test whether gss-proxy is up without waiting.

Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>