x86/mm: Remove redundant check for kmem_cache_create()

The flag 'SLAB_PANIC' implies panic on failure, So there is no need to
check the returned pointer for NULL.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/1528804132-154948-1-git-send-email-cgxu519@gmx.com