CIFS: Fix NULL ptr deref

cifs->master_tlink is NULL against Win Server 2016 (which is
strange.. not sure why) and is dereferenced in cifs_sb_master_tcon().

move master_tlink getter to cifsglob.h so it can be used from
smb2misc.c
Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>

fs/cifs/cifsglob.h

@@ -1019,6 +1019,12 @@ tlink_tcon(struct tcon_link *tlink)
 	return tlink->tl_tcon;
 }
 
+static inline struct tcon_link *
+cifs_sb_master_tlink(struct cifs_sb_info *cifs_sb)
+{
+	return cifs_sb->master_tlink;
+}
+
 extern void cifs_put_tlink(struct tcon_link *tlink);
 
 static inline struct tcon_link *

fs/cifs/connect.c

@@ -3116,12 +3116,6 @@ cifs_put_tlink(struct tcon_link *tlink)
 	return;
 }
 
-static inline struct tcon_link *
-cifs_sb_master_tlink(struct cifs_sb_info *cifs_sb)
-{
-	return cifs_sb->master_tlink;
-}
-
 static int
 compare_mount_options(struct super_block *sb, struct cifs_mnt_data *mnt_data)
 {

fs/cifs/smb2misc.c

@@ -453,8 +453,10 @@ cifs_convert_path_to_utf16(const char *from, struct cifs_sb_info *cifs_sb)
 		start_of_path = from + 1;
 #ifdef CONFIG_CIFS_SMB311
 	/* SMB311 POSIX extensions paths do not include leading slash */
-	else if (cifs_sb_master_tcon(cifs_sb)->posix_extensions)
+	else if (cifs_sb_master_tlink(cifs_sb) &&
+		 cifs_sb_master_tcon(cifs_sb)->posix_extensions) {
 		start_of_path = from + 1;
+	}
 #endif /* 311 */
 	else
 		start_of_path = from;