abd: add page iterator

The regular ABD iterators yield data buffers, so they have to map and
unmap pages into kernel memory. If the caller only wants to count
chunks, or can use page pointers directly, then the map/unmap is just
unnecessary overhead.

This adds adb_iterate_page_func, which yields unmapped struct page
instead.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #15533
Closes #15588
(cherry picked from commit 390b448726)

module/zfs/abd.c

@@ -826,6 +826,48 @@ abd_iterate_func(abd_t *abd, size_t off, size_t size,
 	return (ret);
 }
 
+#if defined(__linux__) && defined(_KERNEL)
+int
+abd_iterate_page_func(abd_t *abd, size_t off, size_t size,
+    abd_iter_page_func_t *func, void *private)
+{
+	struct abd_iter aiter;
+	int ret = 0;
+
+	if (size == 0)
+		return (0);
+
+	abd_verify(abd);
+	ASSERT3U(off + size, <=, abd->abd_size);
+
+	abd_t *c_abd = abd_init_abd_iter(abd, &aiter, off);
+
+	while (size > 0) {
+		IMPLY(abd_is_gang(abd), c_abd != NULL);
+
+		abd_iter_page(&aiter);
+
+		size_t len = MIN(aiter.iter_page_dsize, size);
+		ASSERT3U(len, >, 0);
+
+		ret = func(aiter.iter_page, aiter.iter_page_doff,
+		    len, private);
+
+		aiter.iter_page = NULL;
+		aiter.iter_page_doff = 0;
+		aiter.iter_page_dsize = 0;
+
+		if (ret != 0)
+			break;
+
+		size -= len;
+		c_abd = abd_advance_abd_iter(abd, c_abd, &aiter, len);
+	}
+
+	return (ret);
+}
+#endif
+
 struct buf_arg {
 	void *arg_buf;
 };