sysfs: reimplement sysfs_drop_dentry()

This patch reimplements sysfs_drop_dentry() such that remove_dir() can
use it to drop dentry instead of using a separate mechanism.  With
this change, making directories reclaimable is much easier.

This patch used to contain fixes for two race conditions around
sd->s_dentry but that part has been separated out and included into
mainline early as commit 6aa054aadfea613a437ad0b15d38eca2b963fc0a and
dd14cbc994709a1c5a64ed3621f583c49a27e521.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/fs/sysfs/sysfs.h b/fs/sysfs/sysfs.h
index 9fa77d6..fc6aa86 100644
--- a/fs/sysfs/sysfs.h
+++ b/fs/sysfs/sysfs.h
@@ -76,7 +76,7 @@
 extern int sysfs_create_subdir(struct kobject *, const char *, struct dentry **);
 extern void sysfs_remove_subdir(struct dentry *);
 
-extern void sysfs_drop_dentry(struct sysfs_dirent *sd, struct dentry *parent);
+extern void sysfs_drop_dentry(struct sysfs_dirent *sd);
 extern int sysfs_setattr(struct dentry *dentry, struct iattr *iattr);
 
 extern spinlock_t sysfs_lock;