[PATCH] Andrea Arcangeli: avoid size_buffers_type overflow

[PATCH] Andrea Arcangeli: avoid size_buffers_type overflow

size_buffers_type array, which is an unsigned long, can overflow
on 32-bits: its perfectly possible for PAE machines to have more
than 4Gb of data mapped by buffer_head's at the same time.

Avoid that by accounting 1/512 of the real size (size >> 9).