block: don't check for BIO_MAX_PAGES in blk_bio_segment_split()

blk_bio_segment_split() makes sure bios have no more than
BIO_MAX_PAGES entries in the bi_io_vec.
This was done because bio_clone_bioset() (when given a
mempool bioset) could not handle larger io_vecs.

No driver uses bio_clone_bioset() any more, they all
use bio_clone_fast() if anything, and bio_clone_fast()
doesn't clone the bi_io_vec.

The main user of of bio_clone_bioset() at this level
is bounce.c, and bouncing now happens before blk_bio_segment_split(),
so that is not of concern.

So remove the big helpful comment and the code.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -108,24 +108,8 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 	bool do_split = true;
 	struct bio *new = NULL;
 	const unsigned max_sectors = get_max_io_size(q, bio);
-	unsigned bvecs = 0;
 
 	bio_for_each_segment(bv, bio, iter) {
-		/*
-		 * With arbitrary bio size, the incoming bio may be very
-		 * big. We have to split the bio into small bios so that
-		 * each holds at most BIO_MAX_PAGES bvecs because
-		 * bio_clone_bioset() can fail to allocate big bvecs.
-		 *
-		 * Those drivers which will need to use bio_clone_bioset()
-		 * should tell us in some way.  For now, impose the
-		 * BIO_MAX_PAGES limit on all queues.
-		 *
-		 * TODO: handle users of bio_clone_bioset() differently.
-		 */
-		if (bvecs++ >= BIO_MAX_PAGES)
-			goto split;
-
 		/*
 		 * If the queue doesn't support SG gaps and adding this
 		 * offset would create a gap, disallow it.