of HASH_DELETE().

HASH_ASSERT_VALID(), HASH_INVALIDATE(): New macros, used in HASH_DELETE()
and HASH_SEARCH().

buf_page_init_low(): Add HASH_INVALIDATE(bpage, hash);

buf_page_init(): Call buf_page_init_low() before HASH_INSERT(), so that
bpage->hash will not be invalidated.