net/dns/resolver: set maxDoHInFlight to 1000 on iOS 15+.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

hostinfo/hostinfo.go

@@ -28,7 +28,7 @@ func New() *tailcfg.Hostinfo {
 		IPNVersion:  version.Long,
 		Hostname:    hostname,
 		OS:          version.OS(),
-		OSVersion:   getOSVersion(),
+		OSVersion:   GetOSVersion(),
 		Package:     packageType(),
 		GoArch:      runtime.GOARCH,
 		DeviceModel: deviceModel(),
@@ -37,7 +37,8 @@ func New() *tailcfg.Hostinfo {
 
 var osVersion func() string // non-nil on some platforms
 
-func getOSVersion() string {
+// GetOSVersion returns the OSVersion of current host if available.
+func GetOSVersion() string {
 	if s, _ := osVersionAtomic.Load().(string); s != "" {
 		return s
 	}