all: use cmpx.Or where it made sense

I left a few out where writing it explicitly was better
for various reasons.

Updates #8296

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/linuxfw/nftables.go

@@ -16,6 +16,7 @@ import (
 	"github.com/josharian/native"
 	"golang.org/x/sys/unix"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/cmpx"
 )
 
 // DebugNetfilter prints debug information about netfilter rules to the
@@ -52,27 +53,18 @@ func DebugNetfilter(logf logger.Logf) error {
 			for _, ex := range rule.Exprs {
 				switch v := ex.(type) {
 				case *expr.Meta:
-					key := metaKeyNames[v.Key]
-					if key == "" {
-						key = "UNKNOWN"
-					}
+					key := cmpx.Or(metaKeyNames[v.Key], "UNKNOWN")
 					logf("netfilter:     Meta: key=%s source_register=%v register=%d", key, v.SourceRegister, v.Register)
 
 				case *expr.Cmp:
-					op := cmpOpNames[v.Op]
-					if op == "" {
-						op = "UNKNOWN"
-					}
+					op := cmpx.Or(cmpOpNames[v.Op], "UNKNOWN")
 					logf("netfilter:     Cmp: op=%s register=%d data=%s", op, v.Register, formatMaybePrintable(v.Data))
 
 				case *expr.Counter:
 					// don't print
 
 				case *expr.Verdict:
-					kind := verdictNames[v.Kind]
-					if kind == "" {
-						kind = "UNKNOWN"
-					}
+					kind := cmpx.Or(verdictNames[v.Kind], "UNKNOWN")
 					logf("netfilter:     Verdict: kind=%s data=%s", kind, v.Chain)
 
 				case *expr.Target: