cifs: Add helper function to check smb1+ server

SMB1 server's header_preamble_size is not 0, add use is_smb1 function
to simplify the code, no actual functional changes.

Reviewed-by: Paulo Alcantara (SUSE) <[e-mail]>
Signed-off-by: Zhang Xiaoxu <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/cifsencrypt.c

···
 	int rc;
 	struct kvec *iov = rqst->rq_iov;
 	int n_vec = rqst->rq_nvec;
-	bool is_smb2 = HEADER_PREAMBLE_SIZE(server) == 0;
 
 	/* iov[0] is actual data and not the rfc1002 length for SMB2+ */
-	if (is_smb2) {
+	if (!is_smb1(server)) {
 		if (iov[0].iov_len <= 4)
 			return -EIO;
 		i = 0;

fs/cifs/cifsglob.h

···
 #endif
 };
 
+static inline bool is_smb1(struct TCP_Server_Info *server)
+{
+	return HEADER_PREAMBLE_SIZE(server) != 0;
+}
+
 static inline void cifs_server_lock(struct TCP_Server_Info *server)
 {
 	unsigned int nofs_flag = memalloc_nofs_save();

fs/cifs/connect.c

···
 	/*
 	 * SMB1 does not use credits.
 	 */
-	if (HEADER_PREAMBLE_SIZE(server))
+	if (is_smb1(server))
 		return 0;
 
 	return le16_to_cpu(shdr->CreditRequest);
···
 	/*
 	 * SMB1 does not use credits.
 	 */
-	if (HEADER_PREAMBLE_SIZE(server))
+	if (is_smb1(server))
 		return;
 
 	if (shdr->CreditRequest) {
···
 		if (length < 0)
 			continue;
 
-		if (HEADER_PREAMBLE_SIZE(server) == 0)
-			server->total_read = 0;
-		else
+		if (is_smb1(server))
 			server->total_read = length;
+		else
+			server->total_read = 0;
 
 		/*
 		 * The right amount was read from socket - 4 bytes,

fs/cifs/transport.c

···
 	int nvec;
 	unsigned long buflen = 0;
 
-	if (HEADER_PREAMBLE_SIZE(server) == 0 && rqst->rq_nvec >= 2 &&
+	if (!is_smb1(server) && rqst->rq_nvec >= 2 &&
 	    rqst->rq_iov[0].iov_len == 4) {
 		iov = &rqst->rq_iov[1];
 		nvec = rqst->rq_nvec - 1;
···
 	sigprocmask(SIG_BLOCK, &mask, &oldmask);
 
 	/* Generate a rfc1002 marker for SMB2+ */
-	if (HEADER_PREAMBLE_SIZE(server) == 0) {
+	if (!is_smb1(server)) {
 		struct kvec hiov = {
 			.iov_base = &rfc1002_marker,
 			.iov_len  = 4