configfs: unhash the dentry before dropping the item in rmdir

In the Linux kernel, the following vulnerability has been resolved:

configfs: unhash the dentry before dropping the item in rmdir

configfs_get_config_item() treats a hashed dentry as proof that
sd->s_element is a live config_item. configfs_rmdir() breaks that:
simple_rmdir() leaves the dentry hashed, the last reference to the item is
dropped right after, and the dentry is only unhashed by d_delete() once
->rmdir() has returned. configfs_symlink() resolves its target holding no
lock on it, so get_target() can land in that window:

BUG: KASAN: slab-use-after-free in config_item_get+0x26/0x90
get_target fs/configfs/symlink.c:128 [inline]
configfs_symlink+0x4ab/0x1030 fs/configfs/symlink.c:185

Unhash in configfs_remove_dir(), while the item is still guaranteed to be
there. A reference obtained just before that stays harmless, as
create_link() rechecks CONFIGFS_USET_DROPPING, already set by
configfs_detach_prep(). Both configfs_unregister_subsystem() paths
d_drop() after detaching, so this only makes rmdir match them.