This allows us to allocate from either side of the block pool in a
consistent way. If you use the previous block_pool_alloc function, you
will get offsets from the start of the pool as normal. If you use the new
block_pool_alloc_back function, you will get a negative index that
corresponds to something in the "back" of the pool.
We don't have any locking issues yet because we use the pool size itself as
a mutex in block_pool_alloc to guarantee that only one thread is resizing
at a time. However, we are about to add support for growing the block pool
at both ends. This introduces two potential races:
1) You could have two block_pool_alloc() calls that both try to grow the
block pool, one from each end.
2) The relocation handling code will now have to think about not only the
bo that we use for the block pool but also the offset from the start of
that bo to the center of the block pool. It's possible that the block
pool growing code could race with the relocation handling code and get
a bo and offset out of sync.
Grabbing the device mutex solves both of these problems. Thanks to (2), we
can't really do anything more granular.
