rtnl_lock() is a "Big Kernel Lock" in the networking slow path
and still serialises most of RTM_(NEW|DEL|SET)* rtnetlink requests.

Commit 76aed95319da ("rtnetlink: Add per-netns RTNL.") started a
very large, in-progress, effort to make the RTNL lock scope per
network namespace.

However, there are still some patches that newly use rtnl_lock(),
which is now discouraged, and we need to revisit it later.

Let's warn about the case by checkpatch.

The target functions are as follows:

  * rtnl_lock()
  * rtnl_trylock()
  * rtnl_lock_interruptible()
  * rtnl_lock_killable()

and the warning will be like:

  WARNING: A new use of rtnl_lock() variants is discouraged, try to use rtnl_net_lock(net) variants
  #18: FILE: net/core/rtnetlink.c:79:
  +	rtnl_lock();

Signed-off-by: Kuniyuki Iwashima <redacted>
---
It would be nice if this patch goes through net-next.git to catch
new rtnl_lock() users by netdev CI.
---
 scripts/checkpatch.pl | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 7b28ad331742..09d5420436cc 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl

@@ -6995,6 +6995,12 @@ sub process {
 #			}
 #		}
 
+# A new use of rtnl_lock() is discouraged as it's being converted to rtnl_net_lock(net).
+		if ($line =~ /^\+.*\brtnl_(try)?lock(_interruptible|_killable)?\(\)/) {