SUNRPC: Don't attempt to destroy expired RPCSEC_GSS credentials..

..and always destroy using a 'soft' RPC call. Destroying GSS credentials
isn't mandatory; the server can always cope with a few credentials not
getting destroyed in a timely fashion.

This actually fixes a hang situation. Basically, some servers will decide
that the client is crazy if it tries to destroy an RPC context for which
they have sent an RPCSEC_GSS_CREDPROBLEM, and so will refuse to talk to it
for a while.
The regression therefor probably was introduced by commit
0df7fb74fbb709591301871a38aac7735a1d6583.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>