memtest: use kstrtouint instead of simple_strtoul

Since simple_strtoul is obsolete and memtest_pattern is type of int, use
kstrtouint instead.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Cc: Leon Romanovsky <leon@leon.nu>
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>