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>