rtnl: reset calcit fptr in rtnl_unregister()

BugLink: https://bugs.launchpad.net/bugs/1878232

commit f567e950 upstream.

To avoid having dangling function pointers left behind, reset calcit in
rtnl_unregister(), too.

This is no issue so far, as only the rtnl core registers a netlink
handler with a calcit hook which won't be unregistered, but may become
one if new code makes use of the calcit hook.

Fixes: c7ac8679 ("rtnetlink: Compute and store minimum ifinfo...")
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/core/rtnetlink.c

@@ -253,6 +253,7 @@ int rtnl_unregister(int protocol, int msgtype)
 
 	rtnl_msg_handlers[protocol][msgindex].doit = NULL;
 	rtnl_msg_handlers[protocol][msgindex].dumpit = NULL;
+	rtnl_msg_handlers[protocol][msgindex].calcit = NULL;
 
 	return 0;
 }