checkpatch: warn about uses of ENOTSUPP

ENOTSUPP often feels like the right error code to use, but it's
in fact not a standard Unix error. E.g.:

$ python
>>> import errno
>>> errno.errorcode[errno.ENOTSUPP]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'errno' has no attribute 'ENOTSUPP'

There were numerous commits converting the uses back to EOPNOTSUPP
but in some cases we are stuck with the high error code for backward
compatibility reasons.

Let's try prevent more ENOTSUPPs from getting into the kernel.

Recent example:
https://lore.kernel.org/netdev/20200510182252.GA411829@lunn.ch/

v3 (Joe):
 - fix the "not file" condition.

v2 (Joe):
 - add a link to recent discussion,
 - don't match when scanning files, not patches to avoid sudden
   influx of conversion patches.
https://lore.kernel.org/netdev/20200511165319.2251678-1-kuba@kernel.org/

v1:
https://lore.kernel.org/netdev/20200510185148.2230767-1-kuba@kernel.org/Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

scripts/checkpatch.pl

@@ -4199,6 +4199,17 @@ sub process {
 			     "ENOSYS means 'invalid syscall nr' and nothing else\n" . $herecurr);
 		}
 
+# ENOTSUPP is not a standard error code and should be avoided in new patches.
+# Folks usually mean EOPNOTSUPP (also called ENOTSUP), when they type ENOTSUPP.
+# Similarly to ENOSYS warning a small number of false positives is expected.
+		if (!$file && $line =~ /\bENOTSUPP\b/) {
+			if (WARN("ENOTSUPP",
+				 "ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP\n" . $herecurr) &&
+			    $fix) {
+				$fixed[$fixlinenr] =~ s/\bENOTSUPP\b/EOPNOTSUPP/;
+			}
+		}
+
 # function brace can't be on same line, except for #defines of do while,
 # or if closed on same line
 		if ($perl_version_ok &&