sysfs: make sysfs_drop_dentry() access inodes using ilookup()

sysfs_drop_dentry() used to go through sd->s_dentry and
sd->s_parent->s_dentry to access the inodes.  This is incorrect
because inode can be cached without dentry.

This patch makes sysfs_drop_dentry() access inodes using ilookup() on
sd->s_ino.  This is both correct and simpler.

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