bio: skip atomic inc/dec of ->bi_cnt for most use cases

Struct bio has a reference count that controls when it can be freed.
Most uses cases is allocating the bio, which then returns with a
single reference to it, doing IO, and then dropping that single
reference. We can remove this atomic_dec_and_test() in the completion
path, if nobody else is holding a reference to the bio.

If someone does call bio_get() on the bio, then we flag the bio as
now having valid count and that we must properly honor the reference
count when it's being put.

Tested-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/xfs/xfs_aops.c

@@ -356,7 +356,6 @@ xfs_end_bio(
 {
 	xfs_ioend_t		*ioend = bio->bi_private;
 
-	ASSERT(atomic_read(&bio->bi_cnt) >= 1);
 	ioend->io_error = test_bit(BIO_UPTODATE, &bio->bi_flags) ? 0 : error;
 
 	/* Toss bio and pass work off to an xfsdatad thread */