mm/page_alloc: fallbacks at most has 3 elements

MIGRAGE_TYPES is used to be the mark of end and there are at most 3
elements for the one dimension array.

Reduce to 3 to save little memory.

Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: http://lkml.kernel.org/r/20200625231022.18784-1-richard.weiyang@linux.alibaba.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>