SUNRPC: Don't leak netobj memory when gss_read_proxy_verf() fails

Fixes: 030d794b ("SUNRPC: Use gssproxy upcall for server RPCGSS authentication.")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>