Add linters and fix lint issues

internal/routing/entry.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"net"
 	"strconv"
-
 	"strings"
 )
 
@@ -28,7 +27,8 @@ func parseRoutingEntry(s string) (r routingEntry, err error) {
 		return fmt.Errorf("line %q: %w", s, err)
 	}
 	fields := strings.Fields(s)
-	if len(fields) < 11 {
+	const minFields = 11
+	if len(fields) < minFields {
 		return r, wrapError(fmt.Errorf("not enough fields"))
 	}
 	r.iface = fields[0]
@@ -74,20 +74,22 @@ func parseRoutingEntry(s string) (r routingEntry, err error) {
 
 func reversedHexToIPv4(reversedHex string) (ip net.IP, err error) {
 	bytes, err := hex.DecodeString(reversedHex)
+	const nBytesRequired = 4
 	if err != nil {
 		return nil, fmt.Errorf("cannot parse reversed IP hex %q: %s", reversedHex, err)
-	} else if len(bytes) != 4 {
-		return nil, fmt.Errorf("hex string contains %d bytes instead of 4", len(bytes))
+	} else if L := len(bytes); L != nBytesRequired {
+		return nil, fmt.Errorf("hex string contains %d bytes instead of %d", L, nBytesRequired)
 	}
 	return []byte{bytes[3], bytes[2], bytes[1], bytes[0]}, nil
 }
 
 func hexToIPv4Mask(hexString string) (mask net.IPMask, err error) {
 	bytes, err := hex.DecodeString(hexString)
+	const nBytesRequired = 4
 	if err != nil {
 		return nil, fmt.Errorf("cannot parse hex mask %q: %s", hexString, err)
-	} else if len(bytes) != 4 {
-		return nil, fmt.Errorf("hex string contains %d bytes instead of 4", len(bytes))
+	} else if L := len(bytes); L != nBytesRequired {
+		return nil, fmt.Errorf("hex string contains %d bytes instead of %d", L, nBytesRequired)
 	}
 	return []byte{bytes[3], bytes[2], bytes[1], bytes[0]}, nil
 }