cifs: when renaming don't try to unlink negative dentry

When attempting to rename a file on a read-only share, the kernel can
call cifs_unlink on a negative dentry, which causes an oops. Only try
to unlink the file if it's a positive dentry.

Signed-off-by: Jeff Layton <[e-mail]>
Tested-by: Shirish Pargaonkar <[e-mail]>
CC: Stable <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/inode.c

···
 		     checking the UniqueId via FILE_INTERNAL_INFO */
 
 unlink_target:
-	if ((rc == -EACCES) || (rc == -EEXIST)) {
+	/* Try unlinking the target dentry if it's not negative */
+	if (target_dentry->d_inode && (rc == -EACCES || rc == -EEXIST)) {
 		tmprc = cifs_unlink(target_dir, target_dentry);
 		if (tmprc)
 			goto cifs_rename_exit;