bcachefs: Fix a memory leak in dio write path

Commit c42bca92 "bio: don't copy bvec
for direct IO" changed bio_iov_iter_get_pages() to point bio->bi_iovec
at the incoming biovec, meaning if we already allocated one, it'll be
leaked.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/fs-io.c

@@ -2025,7 +2025,9 @@ ssize_t bch2_direct_write(struct kiocb *req, struct iov_iter *iter)
 	}
 
 	bio = bio_alloc_bioset(NULL,
-			       iov_iter_npages(iter, BIO_MAX_VECS),
+			       iov_iter_is_bvec(iter)
+			       ? 0
+			       : iov_iter_npages(iter, BIO_MAX_VECS),
 			       REQ_OP_WRITE,
 			       GFP_KERNEL,
 			       &c->dio_write_bioset);