CRED: Wrap task credential accesses in the AX25 protocol

Wrap access to task credentials so that they can be separated more easily from
the task_struct during the introduction of COW creds.

Change most current->(|e|s|fs)[ug]id to current_(|e|s|fs)[ug]id().

Change some task->e?[ug]id to task_e?[ug]id(). In some places it makes more
sense to use RCU directly rather than a convenient wrapper; these will be
addressed by later patches.

Signed-off-by: David Howells <[e-mail]>
Reviewed-by: James Morris <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Acked-by: Ralf Baechle <[e-mail]>
Cc: [e-mail]
Signed-off-by: James Morris <[e-mail]>

net/ax25/af_ax25.c

···
 	if (addr->fsa_ax25.sax25_family != AF_AX25)
 		return -EINVAL;
 
-	user = ax25_findbyuid(current->euid);
+	user = ax25_findbyuid(current_euid());
 	if (user) {
 		call = user->call;
 		ax25_uid_put(user);

net/ax25/ax25_route.c

···
 		goto put;
 	}
 
-	user = ax25_findbyuid(current->euid);
+	user = ax25_findbyuid(current_euid());
 	if (user) {
 		ax25->source_addr = user->call;
 		ax25_uid_put(user);