sysfs: avoid kmem_cache_free(NULL)

kmem_cache_free() with NULL is not allowed. But it may happen
if out of memory error is triggered in sysfs_new_dirent().
This patch fixes that error handling.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>