SUNRPC: Obscure Kerberos session key

ctx->Ksess is never used after import has completed. Obscure it
immediately so it cannot be re-used or copied.
Tested-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/auth_gss/gss_krb5_mech.c

@@ -550,6 +550,7 @@ gss_import_sec_context_kerberos(const void *p, size_t len,
 		ret = gss_import_v1_context(p, end, ctx);
 	else
 		ret = gss_import_v2_context(p, end, ctx, gfp_mask);
+	memzero_explicit(&ctx->Ksess, sizeof(ctx->Ksess));
 	if (ret) {
 		kfree(ctx);
 		return ret;