cifs: check for private_data before trying to put it

cifs_close doesn't check that the filp->private_data is non-NULL before
trying to put it. That can cause an oops in certain error conditions
that can occur on open or lookup before the private_data is set.
Reported-by: Ben Greear <greearb@candelatech.com>
CC: Stable <stable@kernel.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/file.c

@@ -575,8 +575,10 @@ static int cifs_reopen_file(struct cifsFileInfo *pCifsFile, bool can_flush)
 
 int cifs_close(struct inode *inode, struct file *file)
 {
-	cifsFileInfo_put(file->private_data);
-	file->private_data = NULL;
+	if (file->private_data != NULL) {
+		cifsFileInfo_put(file->private_data);
+		file->private_data = NULL;
+	}
 
 	/* return code from the ->release op is always ignored */
 	return 0;