mm/kmemleak: increase the max mem pool to 1M

There are some machines with slow disk and fast CPUs.  When they are under
memory pressure, it could take a long time to swap before the OOM kicks in
to free up some memory.  As the results, it needs a large mem pool for
kmemleak or suffering from higher chance of a kmemleak metadata allocation
failure.  524288 proves to be the good number for all architectures here.
Increase the upper bound to 1M to leave some room for the future.

Link: http://lkml.kernel.org/r/1565807572-26041-1-git-send-email-cai@lca.pw


Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>