sysfs-writable charp arguments need to be locked against modification
(since the old ones may be kfreed underneath us).  String arguments
are much simpler, so use them for small strings (eg. IFNAMSIZ).

lkdtm only uses the parameters at module initialization time, so there's
not much point making them writable.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: M. Mohan Kumar <mohan@in.ibm.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Julia Lawall <julia@diku.dk>
Cc: devel@driverdev.osuosl.org