block: bio: kill BIO_MAX_SIZE

No one need this macro now, so remove it. Basically
only how many bvecs in one bio matters instead
of how many bytes in this bio.

The motivation is for supporting multipage bvecs, in
which we only know what the max count of bvecs is supported
in the bio.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>