Revert "io_uring/rsrc: disallow multi-source reg buffers"

This reverts commit edd47826.

There's really no specific need to disallow multiple sources of buffers,
and io_uring really should not be mandating this by itself. We should
be able to solely rely on GUP making these decisions.

As this also stands in the way of a cleanup where io_uring is the odd
one out, kill it.

Link: https://lore.kernel.org/all/61ded378-51a8-1dcb-b631-fda1903248a9@gmail.com/Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rsrc.c

@@ -1053,17 +1053,14 @@ struct page **io_pin_pages(unsigned long ubuf, unsigned long len, int *npages)
 	pret = pin_user_pages(ubuf, nr_pages, FOLL_WRITE | FOLL_LONGTERM,
 			      pages, vmas);
 	if (pret == nr_pages) {
-		struct file *file = vmas[0]->vm_file;
-
 		/* don't support file backed memory */
 		for (i = 0; i < nr_pages; i++) {
-			if (vmas[i]->vm_file != file) {
-				ret = -EINVAL;
-				break;
-			}
-			if (!file)
+			struct vm_area_struct *vma = vmas[i];
+
+			if (vma_is_shmem(vma))
 				continue;
-			if (!vma_is_shmem(vmas[i]) && !is_file_hugepages(file)) {
+			if (vma->vm_file &&
+			    !is_file_hugepages(vma->vm_file)) {
 				ret = -EOPNOTSUPP;
 				break;
 			}