mm: slub: avoid return code from slab_sysfs_init if debugfs is off

Avoid returning from slab_sysfs_init if debugfs is off as it is not fatal.

Change-Id: Ice8c26861ad6d6d143e910bb36aeed61dc23cb2f
Signed-off-by: Faiyaz Mohammed <faiyazm@codeaurora.org>