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>

net/dnscache/messagecache.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/golang/groupcache/lru"
 	"golang.org/x/net/dns/dnsmessage"
+	"tailscale.com/util/cmpx"
 )
 
 // MessageCache is a cache that works at the DNS message layer,
@@ -59,10 +60,7 @@ func (c *MessageCache) Flush() {
 // pruneLocked prunes down the cache size to the configured (or
 // default) max size.
 func (c *MessageCache) pruneLocked() {
-	max := c.cacheSizeSet
-	if max == 0 {
-		max = 500
-	}
+	max := cmpx.Or(c.cacheSizeSet, 500)
 	for c.cache.Len() > max {
 		c.cache.RemoveOldest()
 	}