mm/large system hash: use vmalloc for size > MAX_ORDER when !hashdist

The kernel currently clamps large system hashes to MAX_ORDER when hashdist
is not set, which is rather arbitrary.

vmalloc space is limited on 32-bit machines, but this shouldn't result in
much more used because of small physical memory limiting system hash
sizes.

Include "vmalloc" or "linear" in the kernel log message.

Link: http://lkml.kernel.org/r/20190605144814.29319-1-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>