io_uring/rsrc: cleanup io_pin_pages()

This function is overly convoluted with a goto error path, and checks
under the mmap_read_lock() that don't need to be at all. Rearrange it
a bit so the checks and errors fall out naturally, rather than needing
to jump around for it.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rsrc.c

@@ -1037,39 +1037,36 @@ struct page **io_pin_pages(unsigned long ubuf, unsigned long len, int *npages)
 {
 	unsigned long start, end, nr_pages;
 	struct page **pages = NULL;
-	int pret, ret = -ENOMEM;
+	int ret;
 
 	end = (ubuf + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	start = ubuf >> PAGE_SHIFT;
 	nr_pages = end - start;
+	WARN_ON(!nr_pages);
 
 	pages = kvmalloc_array(nr_pages, sizeof(struct page *), GFP_KERNEL);
 	if (!pages)
-		goto done;
+		return ERR_PTR(-ENOMEM);
 
-	ret = 0;
 	mmap_read_lock(current->mm);
-	pret = pin_user_pages(ubuf, nr_pages, FOLL_WRITE | FOLL_LONGTERM,
-			      pages);
-	if (pret == nr_pages)
+	ret = pin_user_pages(ubuf, nr_pages, FOLL_WRITE | FOLL_LONGTERM, pages);
+	mmap_read_unlock(current->mm);
+
+	/* success, mapped all pages */
+	if (ret == nr_pages) {
 		*npages = nr_pages;
-	else
-		ret = pret < 0 ? pret : -EFAULT;
+		return pages;
+	}
 
-	mmap_read_unlock(current->mm);
-	if (ret) {
+	/* partial map, or didn't map anything */
+	if (ret >= 0) {
 		/* if we did partial map, release any pages we did get */
-		if (pret > 0)
-			unpin_user_pages(pages, pret);
-		goto done;
-	}
-	ret = 0;
-done:
-	if (ret < 0) {
-		kvfree(pages);
-		pages = ERR_PTR(ret);
+		if (ret)
+			unpin_user_pages(pages, ret);
+		ret = -EFAULT;
 	}
-	return pages;
+	kvfree(pages);
+	return ERR_PTR(ret);
 }
 
 static int io_sqe_buffer_register(struct io_ring_ctx *ctx, struct iovec *iov,