mm/mem-hotplug: replace simple_strtoull() with kstrtoull()

Use the newer and more pleasant kstrtoull() to replace
simple_strtoull(), because simple_strtoull() is marked for obsoletion.

Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>