bcachefs: Use bio_iov_vecs_to_alloc()

This fixes a bug in the DIO read path where, when using a loopback
device in DIO mode, we'd allocate a biovec that would get overwritten
and leaked in bio_iov_iter_get_pages() -> bio_iov_bvec_set().
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/fs-io.c

@@ -1889,7 +1889,7 @@ static int bch2_direct_IO_read(struct kiocb *req, struct iov_iter *iter)
 	iter->count -= shorten;
 
 	bio = bio_alloc_bioset(NULL,
-			       iov_iter_npages(iter, BIO_MAX_VECS),
+			       bio_iov_vecs_to_alloc(iter, BIO_MAX_VECS),
 			       REQ_OP_READ,
 			       GFP_KERNEL,
 			       &c->dio_read_bioset);
@@ -1926,7 +1926,7 @@ static int bch2_direct_IO_read(struct kiocb *req, struct iov_iter *iter)
 	goto start;
 	while (iter->count) {
 		bio = bio_alloc_bioset(NULL,
-				       iov_iter_npages(iter, BIO_MAX_VECS),
+				       bio_iov_vecs_to_alloc(iter, BIO_MAX_VECS),
 				       REQ_OP_READ,
 				       GFP_KERNEL,
 				       &c->bio_read);
@@ -2297,9 +2297,7 @@ ssize_t bch2_direct_write(struct kiocb *req, struct iov_iter *iter)
 	}
 
 	bio = bio_alloc_bioset(NULL,
-			       iov_iter_is_bvec(iter)
-			       ? 0
-			       : iov_iter_npages(iter, BIO_MAX_VECS),
+			       bio_iov_vecs_to_alloc(iter, BIO_MAX_VECS),
 			       REQ_OP_WRITE,
 			       GFP_KERNEL,
 			       &c->dio_write_bioset);