Pull request: all: rm var shadowing, vol. 2

Updates #2803.

Squashed commit of the following:

commit bf7186378164f19ea9e21ec832526792efa2f9c3
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 19:48:17 2021 +0300

    all: rm var shadowing, vol. 2

internal/dnsfilter/dnsfilter.go

@@ -850,7 +850,7 @@ func New(c *Config, blockFilters []Filter) *DNSFilter {
 	d.BlockedServices = bsvcs
 
 	if blockFilters != nil {
-		err := d.initFiltering(nil, blockFilters)
+		err = d.initFiltering(nil, blockFilters)
 		if err != nil {
 			log.Error("Can't initialize filtering subsystem: %s", err)
 			d.Close()