9pfs: make Error **errp const where it is appropriate

Mostly, Error ** is for returning error from the function, so the
callee sets it. However error_append_security_model_hint and
error_append_socket_sockfd_hint get already filled errp
parameter. They don't change the pointer itself, only change the
internal state of referenced Error object. So we can make it Error
*const * errp, to stress the behavior. It will also help coccinelle
script (in future) to distinguish such cases from common errp usage.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Acked-by: Greg Kurz <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
[Commit message replaced]
Signed-off-by: Markus Armbruster <[email protected]>

Author: Vladimir Sementsov-Ogievskiy

hw/9pfs/9p-local.c

@@ -1473,7 +1473,7 @@ static void local_cleanup(FsContext *ctx)
     g_free(data);
 }
 
-static void error_append_security_model_hint(Error **errp)
+static void error_append_security_model_hint(Error *const *errp)
 {
     error_append_hint(errp, "Valid options are: security_model="
                       "[passthrough|mapped-xattr|mapped-file|none]\n");

hw/9pfs/9p-proxy.c

@@ -1114,7 +1114,7 @@ static int connect_namedsocket(const char *path, Error **errp)
     return sockfd;
 }
 
-static void error_append_socket_sockfd_hint(Error **errp)
+static void error_append_socket_sockfd_hint(Error *const *errp)
 {
     error_append_hint(errp, "Either specify socket=/some/path where /some/path"
                       " points to a listening AF_UNIX socket or sock_fd=fd"