configfs: fix config_item refcnt leak in configfs_rmdir()

configfs_rmdir() invokes configfs_get_config_item(), which returns a
reference of the specified config_item object to "parent_item" with
increased refcnt.

When configfs_rmdir() returns, local variable "parent_item" becomes
invalid, so the refcount should be decreased to keep refcount balanced.

The reference counting issue happens in one exception handling path of
configfs_rmdir(). When down_write_killable() fails, the function forgets
to decrease the refcnt increased by configfs_get_config_item(), causing
a refcnt leak.

Fix this issue by calling config_item_put() when down_write_killable()
fails.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>