checkpatch: make --strict a default for files in drivers/net and net/

Networking files are generally more strictly conformant to linux-kernel
style so make checkpatch more verbose by default for patches to files or
when checking files in these directories.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>