SUNRPC: Rename automatic variables in svcauth_gss_wrap_resp_priv()

Clean up variable names to match the other unwrap and wrap
functions.

Additionally, the explicit type cast on @gsd in unnecessary; and
@resbuf is renamed to match the variable naming in the unwrap
functions.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>