CRED: Wrap task credential accesses in the block loopback driver

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 <dhowells@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: James Morris <jmorris@namei.org>

drivers/block/loop.c

@@ -936,8 +936,10 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 {
 	int err;
 	struct loop_func_table *xfer;
+	uid_t uid = current_uid();
 
-	if (lo->lo_encrypt_key_size && lo->lo_key_owner != current->uid &&
+	if (lo->lo_encrypt_key_size &&
+	    lo->lo_key_owner != uid &&
 	    !capable(CAP_SYS_ADMIN))
 		return -EPERM;
 	if (lo->lo_state != Lo_bound)
@@ -992,7 +994,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 	if (info->lo_encrypt_key_size) {
 		memcpy(lo->lo_encrypt_key, info->lo_encrypt_key,
 		       info->lo_encrypt_key_size);
-		lo->lo_key_owner = current->uid;
+		lo->lo_key_owner = uid;
 	}	
 
 	return 0;