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: 72e73c78c446 ("cifs: close the shared root handle on tree disconnect")

Signed-off-by: Steve French <[e-mail]>
Acked-by: Ronnie Sahlberg <[e-mail]>
Reviewed-by: Aurelien Aptel <[e-mail]>
Reviewed-by: Pavel Shilovsky <[e-mail]>
Reported-and-tested-by: Arthur Marsh <[e-mail]>

fs/cifs/smb2pdu.c

···
 	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);