Cosmetic changes to linker allocator and config

Make the num_free_blocks calculation clearer. There are
sizeof(page->bytes) bytes available for blocks, which is equal to
(kAllocateSize - 16). The previous calculation overestimates the amount
of space by 8 or 12 bytes, but that doesn't change the result because
both kAllocateSize and block_size_ are multiples of 16.

Bug: none
Test: boot device, bionic unit tests
Change-Id: Ia880cd5abc2a7bb4e9e6a8c0441d985f4df8fdc4