Slab: fixed small allocations on systems with large pagesize.
Slab: fixed small allocations on systems with large pagesize.

Notably, on ppc64 with 64k pagesize, slab 0 (of size 8) requires
128 64-bit elements for bitmasks.  The code bogusly assumed that
one uintptr_t is enough for bitmasks plus at least one free slot.

hy0kl/nginx,hy0kl/nginx,hy0kl/nginx