btrfs: refactor main loop in copy_extent_buffer_full()

[BACKGROUND]
copy_extent_buffer_full() currently does different handling for regular
and subpage cases, for regular cases it does a page by page copying.
For subpage cases, it just copies the content.

This is fine for the page based extent buffer code, but for the incoming
folio conversion, it can be a burden to add a new branch just to handle
all the different combinations (subpage vs regular, one single folio vs
multi pages).

[ENHANCE]
Instead of handling the different combinations, just go one single
handling for all cases, utilizing write_extent_buffer() to do the
copying.
Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4190,24 +4190,19 @@ void memzero_extent_buffer(const struct extent_buffer *eb, unsigned long start,
 void copy_extent_buffer_full(const struct extent_buffer *dst,
 			     const struct extent_buffer *src)
 {
-	int i;
-	int num_pages;
+	unsigned long cur = 0;
 
 	ASSERT(dst->len == src->len);
 
-	if (dst->fs_info->nodesize >= PAGE_SIZE) {
-		num_pages = num_extent_pages(dst);
-		for (i = 0; i < num_pages; i++)
-			copy_page(page_address(dst->pages[i]),
-				  page_address(src->pages[i]));
-	} else {
-		size_t src_offset = get_eb_offset_in_page(src, 0);
-		size_t dst_offset = get_eb_offset_in_page(dst, 0);
+	while (cur < src->len) {
+		unsigned long index = get_eb_page_index(cur);
+		unsigned long offset = get_eb_offset_in_page(src, cur);
+		unsigned long cur_len = min(src->len, PAGE_SIZE - offset);
+		void *addr = page_address(src->pages[index]) + offset;
+
+		write_extent_buffer(dst, addr, cur, cur_len);
 
-		ASSERT(src->fs_info->nodesize < PAGE_SIZE);
-		memcpy(page_address(dst->pages[0]) + dst_offset,
-		       page_address(src->pages[0]) + src_offset,
-		       src->len);
+		cur += cur_len;
 	}
 }