block: remove the 1 and 4 vec bvec_slabs entries

All bios with up to 4 bvecs use the inline bvecs in the bio itself, so
don't bother to define bvec_slabs entries for them.  Also decruftify
the bvec_slabs definition and initialization while we're at it.

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