cifs: Fix cached_fid refcnt leak in open_shroot

open_shroot() invokes kref_get(), which increases the refcount of the
"tcon->crfid" object. When open_shroot() returns not zero, it means the
open operation failed and close_shroot() will not be called to decrement
the refcount of the "tcon->crfid".

The reference counting issue happens in one normal path of
open_shroot(). When the cached root have been opened successfully in a
concurrent process, the function increases the refcount and jump to
"oshr_free" to return. However the current return value "rc" may not
equal to 0, thus the increased refcount will not be balanced outside the
function, causing a refcnt leak.

Fix this issue by setting the value of "rc" to 0 before jumping to
"oshr_free" label.
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/smb2ops.c

@@ -763,6 +763,7 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon,
 			/* close extra handle outside of crit sec */
 			SMB2_close(xid, tcon, fid.persistent_fid, fid.volatile_fid);
 		}
+		rc = 0;
 		goto oshr_free;
 	}