smb3: fix refcount underflow warning on unmount when no directory leases

Fix refcount underflow warning when unmounting to servers which didn't grant
directory leases.

[  301.680095] refcount_t: underflow; use-after-free.
[  301.680192] WARNING: CPU: 1 PID: 3569 at lib/refcount.c:28
refcount_warn_saturate+0xb4/0xf3
...
[  301.682139] Call Trace:
[  301.682240]  close_shroot+0x97/0xda [cifs]
[  301.682351]  SMB2_tdis+0x7c/0x176 [cifs]
[  301.682456]  ? _get_xid+0x58/0x91 [cifs]
[  301.682563]  cifs_put_tcon.part.0+0x99/0x202 [cifs]
[  301.682637]  ? ida_free+0x99/0x10a
[  301.682727]  ? cifs_umount+0x3d/0x9d [cifs]
[  301.682829]  cifs_put_tlink+0x3a/0x50 [cifs]
[  301.682929]  cifs_umount+0x44/0x9d [cifs]

Fixes: 72e73c7 ("cifs: close the shared root handle on tree disconnect")

Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Reported-and-tested-by: Arthur Marsh <arthur.marsh@internode.on.net>

fs/cifs/smb2pdu.c

@@ -1847,7 +1847,8 @@ SMB2_tdis(const unsigned int xid, struct cifs_tcon *tcon)
 	if ((tcon->need_reconnect) || (tcon->ses->need_reconnect))
 		return 0;
 
-	close_shroot(&tcon->crfid);
+	if (tcon->crfid.is_valid)
+		close_shroot(&tcon->crfid);
 
 	rc = smb2_plain_req_init(SMB2_TREE_DISCONNECT, tcon, (void **) &req,
 			     &total_len);