md/raid5: use md_write_start to count stripes, not bios

We use md_write_start() to increase the count of pending writes, and
md_write_end() to decrement the count.  We currently count bios
submitted to md/raid5.  Change it count stripe_heads that a WRITE bio
has been attached to.

So now, raid5_make_request() calls md_write_start() and then
md_write_end() to keep the count elevated during the setup of the
request.

add_stripe_bio() calls md_write_start() for each stripe_head, and the
completion routines always call md_write_end(), instead of only
calling it when raid5_dec_bi_active_stripes() returns 0.
make_discard_request also calls md_write_start/end().

The parallel between md_write_{start,end} and use of bi_phys_segments
can be seen in that:
 Whenever we set bi_phys_segments to 1, we now call md_write_start.
 Whenever we increment it on non-read requests with
   raid5_inc_bi_active_stripes(), we now call md_write_start().
 Whenever we decrement bi_phys_segments on non-read requsts with
    raid5_dec_bi_active_stripes(), we now call md_write_end().

This reduces our dependence on keeping a per-bio count of active
stripes in bi_phys_segments.

md_write_inc() is added which parallels md_write_start(), but requires
that a write has already been started, and is certain never to sleep.
This can be used inside a spinlocked region when adding to a write
request.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5-cache.c

@@ -318,8 +318,8 @@ r5c_return_dev_pending_writes(struct r5conf *conf, struct r5dev *dev,
 	while (wbi && wbi->bi_iter.bi_sector <
 	       dev->sector + STRIPE_SECTORS) {
 		wbi2 = r5_next_bio(wbi, dev->sector);
+		md_write_end(conf->mddev);
 		if (!raid5_dec_bi_active_stripes(wbi)) {
-			md_write_end(conf->mddev);
 			bio_list_add(return_bi, wbi);
 		}
 		wbi = wbi2;