bio: add allocation cache abstraction

Add a per-cpu bio_set cache for bio allocations, enabling us to quickly
recycle them instead of going through the slab allocator. This cache
isn't IRQ safe, and hence is only really suitable for polled IO.

Very simple - keeps a count of bio's in the cache, and maintains a max
of 512 with a slack of 64. If we get above max + slack, we drop slack
number of bio's.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>