SUNRPC: Clean up svcauth_gss_accept's NULL procedure check

Micro-optimizations:

1. The value of rqstp->rq_auth_stat is replaced no matter which
   arm of the switch is taken, so the initial assignment can be
   safely removed.

2. Avoid checking the value of gc->gc_proc twice in the I/O
   (RPC_GSS_PROC_DATA) path.

The cost is a little extra code redundancy.

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