cifs: check for smb1 in open_cached_dir()

Check protocol version in open_cached_dir() and return not supported
for SMB1.  This allows us to call open_cached_dir() from code that
is common to both smb1 and smb2/3 in future patches without having to
do this check in the call-site.
At the same time, add a check if tcon is valid or not for the same reason.
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2ops.c

@@ -776,7 +776,8 @@ int open_cached_dir(unsigned int xid, struct cifs_tcon *tcon,
 	struct cifs_fid *pfid;
 	struct dentry *dentry;
 
-	if (tcon->nohandlecache)
+	if (tcon == NULL || tcon->nohandlecache ||
+	    is_smb1_server(tcon->ses->server))
 		return -ENOTSUPP;
 
 	if (cifs_sb->root == NULL)