smb3: if server does not support posix do not allow posix mount option

If user specifies "posix" on an SMB3.11 mount, then fail the mount
if server does not return the POSIX negotiate context indicating
support for posix.
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>

fs/cifs/connect.c

@@ -3031,11 +3031,15 @@ cifs_get_tcon(struct cifs_ses *ses, struct smb_vol *volume_info)
 	}
 
 #ifdef CONFIG_CIFS_SMB311
-	if ((volume_info->linux_ext) && (ses->server->posix_ext_supported)) {
-		if (ses->server->vals->protocol_id == SMB311_PROT_ID) {
+	if (volume_info->linux_ext) {
+		if (ses->server->posix_ext_supported) {
 			tcon->posix_extensions = true;
 			printk_once(KERN_WARNING
 				"SMB3.11 POSIX Extensions are experimental\n");
+		} else {
+			cifs_dbg(VFS, "Server does not support mounting with posix SMB3.11 extensions.\n");
+			rc = -EOPNOTSUPP;
+			goto out_fail;
 		}
 	}
 #endif /* 311 */