block: memory allocations in bounce_clone_bio must not fail

The caller can't cope with a failure from bounce_clone_bio, so
use __GFP_NOFAIL for the passthrough case.  bio_alloc_bioset already
won't fail due to the use of mempools.

And yes, we need to get rid of this bock layer bouncing code entirely
sooner or later..
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bounce.c

@@ -242,12 +242,11 @@ static struct bio *bounce_clone_bio(struct bio *bio_src)
 	 *    __bio_clone_fast() anyways.
 	 */
 	if (bio_is_passthrough(bio_src))
-		bio = bio_kmalloc(GFP_NOIO, bio_segments(bio_src));
+		bio = bio_kmalloc(GFP_NOIO | __GFP_NOFAIL,
+				  bio_segments(bio_src));
 	else
 		bio = bio_alloc_bioset(GFP_NOIO, bio_segments(bio_src),
 				       &bounce_bio_set);
-	if (!bio)
-		return NULL;
 	bio->bi_bdev		= bio_src->bi_bdev;
 	if (bio_flagged(bio_src, BIO_REMAPPED))
 		bio_set_flag(bio, BIO_REMAPPED);