block: reuse BIO_INLINE_VECS for integrity bvecs

bvec_alloc always uses biovec_slabs, and thus always needs to use the
same number of inline vecs.  Share a single definition for the data
and integrity bvecs.

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