sysfs: reimplement sysfs_drop_dentry()

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 b80c1c9633ef1a29f38bde6b54c72b7a21112bae and
74533daf1daba06f4c74b54d091e44bc61a71202.

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