Fix sftp hanging after file transfer

Modern versions of ssh refuse to connect to a server with a rsa key that
  is less than 2048 and this may change to 3072 or 4096 in the future.
  ed25519 cannot change keysize

ssh.go

@@ -315,12 +315,12 @@ func (s *session) request(ctx context.Context, req *ssh.Request, conn *ssh.Serve
 		if err != nil {
 			return err
 		}
+		defer srv.Close()
 
 		exitCode := uint32(0)
 		if err := srv.Serve(); err != nil {
 			log.Printf("(sftp.Server).Serve(): %v", err)
 			if err == io.EOF {
-				defer srv.Close()
 				log.Printf("sftp client exited session")
 			} else {
 				exitCode = 1