block: fix bio_alloc_bioset slowpath GFP handling

bio_alloc_bioset() first strips __GFP_DIRECT_RECLAIM from the optimistic
fast allocation attempt with try_alloc_gfp(). If that fast path fails,
the slowpath checks saved_gfp to decide whether blocking allocation is
allowed, but then still calls mempool_alloc() with the stripped gfp mask.
That can lead to a NULL bio pointer being passed into bio_init().

Fix the slowpath by using saved_gfp for the bio and bvec mempool
allocations.

Fixes: b520c4e ("block: split bio_alloc_bioset more clearly into a fast and slowpath")
Reported-by: syzbot+09ddb593eea76a158f42@syzkaller.appspotmail.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/p01.gc6e9ad5845ad.ttca29g@ub.hpns
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Vasily Gorbik

block/bio.c

@@ -581,11 +581,11 @@ struct bio *bio_alloc_bioset(struct block_device *bdev, unsigned short nr_vecs,
 		 */
 		opf &= ~REQ_ALLOC_CACHE;
 
-		p = mempool_alloc(&bs->bio_pool, gfp);
+		p = mempool_alloc(&bs->bio_pool, saved_gfp);
 		bio = p + bs->front_pad;
 		if (nr_vecs > BIO_INLINE_VECS) {
 			nr_vecs = BIO_MAX_VECS;
-			bvecs = mempool_alloc(&bs->bvec_pool, gfp);
+			bvecs = mempool_alloc(&bs->bvec_pool, saved_gfp);
 		}
 	}