smb3: fix problem unloading module due to leaked refcount on shutdown

The shutdown ioctl can leak a refcount on the tlink which can
prevent rmmod (unloading the cifs.ko) module from working.

Found while debugging xfstest generic/043

Fixes: 69ca1f5755 ("smb3: add dynamic tracepoints for shutdown ioctl")
Reviewed-by: Meetakshi Setiya <msetiya@microsoft.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/connect.c

@@ -4194,6 +4194,9 @@ tlink_rb_insert(struct rb_root *root, struct tcon_link *new_tlink)
  *
  * If one doesn't exist then insert a new tcon_link struct into the tree and
  * try to construct a new one.
+ *
+ * REMEMBER to call cifs_put_tlink() after successful calls to cifs_sb_tlink,
+ * to avoid refcount issues
  */
 struct tcon_link *
 cifs_sb_tlink(struct cifs_sb_info *cifs_sb)

fs/smb/client/ioctl.c

@@ -229,9 +229,11 @@ static int cifs_shutdown(struct super_block *sb, unsigned long arg)
 
 shutdown_good:
 	trace_smb3_shutdown_done(flags, tcon->tid);
+	cifs_put_tlink(tlink);
 	return 0;
 shutdown_out_err:
 	trace_smb3_shutdown_err(rc, flags, tcon->tid);
+	cifs_put_tlink(tlink);
 	return rc;
 }
 

fs/smb/client/link.c

@@ -588,6 +588,7 @@ cifs_symlink(struct mnt_idmap *idmap, struct inode *inode,
 	tlink = cifs_sb_tlink(cifs_sb);
 	if (IS_ERR(tlink)) {
 		rc = PTR_ERR(tlink);
+		/* BB could be clearer if skipped put_tlink on error here, but harmless */
 		goto symlink_exit;
 	}
 	pTcon = tlink_tcon(tlink);