checkpatch: Discourage a new use of rtnl_lock() variants.

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

Commit 76aed95 ("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
  kernel-patches#18: FILE: net/core/rtnetlink.c:79:
  +	rtnl_lock();

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: NipaLocal <nipa@local>

Author: q2ven

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)?\(\)/) {
+			WARN("rtnl_lock()",
+			     "A new use of rtnl_lock() variants is discouraged, try to use rtnl_net_lock(net) variants\n" . $herecurr);
+		}
+
 # strcpy uses that should likely be strscpy
 		if ($line =~ /\bstrcpy\s*\(/) {
 			WARN("STRCPY",