netdevsim: Ensure policer drop counter always increases

In case the policer drop counter is retrieved when the jiffies value is
a multiple of 64, the counter will not be incremented.

This randomly breaks a selftest [1] the reads the counter twice and
checks that it was incremented:

```
TEST: Trap policer [FAIL]
Policer drop counter was not incremented
```

Fix by always incrementing the counter by 1.

[1] tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh

Fixes: ad188458d012 ("netdevsim: Add devlink-trap policer support")
Signed-off-by: Ido Schimmel <[e-mail]>
Reviewed-by: Jiri Pirko <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/netdevsim/dev.c

···
 		return -EINVAL;
 
 	cnt = &nsim_dev->trap_data->trap_policers_cnt_arr[policer->id - 1];
-	*p_drops = *cnt;
-	*cnt += jiffies % 64;
+	*p_drops = (*cnt)++;
 
 	return 0;
 }