sunrpc: fix potential race between setting use_gss_proxy and the upcall rpc_clnt

An nfsd thread can call use_gss_proxy and find it set to '1' but find
gssp_clnt still NULL, so that when it attempts the upcall the result
will be an unnecessary -EIO.

So, ensure that gssp_clnt is created first, and set the use_gss_proxy
variable only if that succeeds.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>