keys: Pass the network namespace into request_key mechanism

Create a request_key_net() function and use it to pass the network
namespace domain tag into DNS revolver keys and rxrpc/AFS keys so that keys
for different domains can coexist in the same keyring.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: netdev@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-afs@lists.infradead.org

include/linux/dns_resolver.h

@@ -26,7 +26,8 @@
 
 #include <uapi/linux/dns_resolver.h>
 
-extern int dns_query(const char *type, const char *name, size_t namelen,
+struct net;
+extern int dns_query(struct net *net, const char *type, const char *name, size_t namelen,
 		     const char *options, char **_result, time64_t *_expiry,
 		     bool invalidate);