block: Kill bio_segments()/bi_vcnt usage

When we start sharing biovecs, keeping bi_vcnt accurate for splits is
going to be error prone - and unnecessary, if we refactor some code.

So bio_segments() has to go - but most of the existing users just needed
to know if the bio had multiple segments, which is easier - add a
bio_multiple_segments() for them.

(Two of the current uses of bio_segments() are going to go away in a
couple patches, but the current implementation of bio_segments() is
unsafe as soon as we start doing driver conversions for immutable
biovecs - so implement a dumb version for bisectability, it'll go away
in a couple patches)

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Neil Brown <neilb@suse.de>
Cc: Nagalakshmi Nandigama <Nagalakshmi.Nandigama@lsi.com>
Cc: Sreekanth Reddy <Sreekanth.Reddy@lsi.com>
Cc: "James E.J. Bottomley" <JBottomley@parallels.com>

drivers/md/raid0.c

@@ -528,7 +528,7 @@ static void raid0_make_request(struct mddev *mddev, struct bio *bio)
 		sector_t sector = bio->bi_iter.bi_sector;
 		struct bio_pair *bp;
 		/* Sanity check -- queue functions should prevent this happening */
-		if (bio_segments(bio) > 1)
+		if (bio_multiple_segments(bio))
 			goto bad_map;
 		/* This is a one page bio that upper layers
 		 * refuse to split for us, so we need to split it.