UNIV_PAGE_SIZE than the default 16384.

MEM_BLOCK_STANDARD_SIZE: Cap to MEM_MAX_ALLOC_IN_BUF when UNIV_PAGE_SIZE
is less than 16384.

ha_create_func(): Invoke mem_heap_create_in_btr_search() with
ut_min(4096, MEM_MAX_ALLOC_IN_BUF) instead of 4096.  The memory
will be allocated from the buffer pool, and with UNIV_PAGE_SIZE
defined to 4096, there would not be any space for the mem_heap
data structure overhead.