rafthttp: replace inline code with existing function

The isMsgApp function implements the judgment of the MsgApp message,
use the isMsgApp function instead.

Signed-off-by: xakdwch <xakdwch5@gmail.com>

server/etcdserver/api/rafthttp/pipeline.go

@@ -103,7 +103,7 @@ func (p *pipeline) handle() {
 			if err != nil {
 				p.status.deactivate(failureType{source: pipelineMsg, action: "write"}, err.Error())
 
-				if m.Type == raftpb.MsgApp && p.followerStats != nil {
+				if isMsgApp(m) && p.followerStats != nil {
 					p.followerStats.Fail()
 				}
 				p.raft.ReportUnreachable(m.To)
@@ -115,7 +115,7 @@ func (p *pipeline) handle() {
 			}
 
 			p.status.activate()
-			if m.Type == raftpb.MsgApp && p.followerStats != nil {
+			if isMsgApp(m) && p.followerStats != nil {
 				p.followerStats.Succ(end.Sub(start))
 			}
 			if isMsgSnap(m) {