and add compressed-only pages to buf_pool->page_hash.

BUF_POOL_ZIP_FOLD(), BUF_POOL_ZIP_FOLD_PTR(), BUF_POOL_ZIP_FOLD_BPAGE():
Macros for computing buf_pool->zip_hash fold values.  Make use of these
in buf0buddy.c.

buf0buddy.c: Replace some ptr == ut_align_down(ptr, ...) in assertions with
!ut_align_offset(ptr, ...).