mm/kmemleak.c: remove obsolete simple_strtoul

Replace the obsolete simple_strtoul() with kstrtoul().

Change-Id: I60951a8d2b92078229254ea23229147fcf24725e
Signed-off-by: Abhijit Pawar <abhi.c.pawar@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>