just one parameter: UNIV_PAGE_SIZE_SHIFT.

UNIV_PAGE_SIZE, BUF_BUDDY_SIZES: Define in terms of UNIV_PAGE_SIZE_SHIFT.

BUF_BUDDY_LOW_SHIFT: New macro, to simplify the definition of BUF_BUDDY_LOW
and BUF_BUDDY_SIZES.

PAGE_ZIP_DIR_SLOT_MASK: Relax the compile-time check.  This bitmask must be
one less than a power of two, and at least UNIV_PAGE_SIZE - 1.