vk/allocator: Solve a data race in anv_block_pool

vk/allocator: Solve a data race in anv_block_pool

The anv_block_pool data structure suffered from the exact same race as the
state pool.  Namely, that the uniqueness of the blocks handed out depends
on the next_block value increasing monotonically.  However, this invariant
did not hold thanks to our block "return" concept.