comedi: drivers: ni_routes: Use strcmp() instead of memcmp()

The family and device comparisons were using memcmp(), but this could
lead to Out-of-bounds reads when the length was larger than the
buffers being compared. Since these appear to always be NUL-terminated
strings, just use strcmp() instead.

This was found with Clang under LTO:

[ 92.405851][    T1] kernel BUG at lib/string_helpers.c:980!
...
[ 92.409141][ T1] RIP: 0010:fortify_panic (fbdev.c:?)
...
[ 92.410056][ T1] ni_assign_device_routes (fbdev.c:?)
[ 92.410056][ T1] ? unittest_enter (fbdev.c:?)
[ 92.410056][ T1] ni_routes_unittest (ni_routes_test.c:?)
[ 92.410056][ T1] ? unittest_enter (fbdev.c:?)
[ 92.410056][ T1] __initstub__kmod_ni_routes_test__505_604_ni_routes_unittest6 (fbdev.c:?)
[ 92.410056][ T1] do_one_initcall (fbdev.c:?)

Link: https://lore.kernel.org/lkml/20220210072821.GD4074@xsang-OptiPlex-9020
Fixes: 4bb90c8 ("staging: comedi: add interface to ni routing table information")
Cc: Ian Abbott <[email protected]>
Cc: H Hartley Sweeten <[email protected]>
Cc: Spencer E. Olson <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Lee Jones <[email protected]>
Reported-by: kernel test robot <[email protected]>
Reviewed-by: Ian Abbott <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/comedi/drivers/ni_routes.c

@@ -56,8 +56,7 @@ static const u8 *ni_find_route_values(const char *device_family)
 	int i;
 
 	for (i = 0; ni_all_route_values[i]; ++i) {
-		if (memcmp(ni_all_route_values[i]->family, device_family,
-			   strnlen(device_family, 30)) == 0) {
+		if (!strcmp(ni_all_route_values[i]->family, device_family)) {
 			rv = &ni_all_route_values[i]->register_values[0][0];
 			break;
 		}
@@ -75,8 +74,7 @@ ni_find_valid_routes(const char *board_name)
 	int i;
 
 	for (i = 0; ni_device_routes_list[i]; ++i) {
-		if (memcmp(ni_device_routes_list[i]->device, board_name,
-			   strnlen(board_name, 30)) == 0) {
+		if (!strcmp(ni_device_routes_list[i]->device, board_name)) {
 			dr = ni_device_routes_list[i];
 			break;
 		}