btrfs: use memset_page instead of opencoding it

Use memset_page() in memset_extent_buffer() instead of opencoding it.

This does not not change any functionality.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4208,7 +4208,7 @@ static void memset_extent_buffer(const struct extent_buffer *eb, int c,
 		struct page *page = eb->pages[index];
 
 		assert_eb_page_uptodate(eb, page);
-		memset(page_address(page) + offset, c, cur_len);
+		memset_page(page, offset, c, cur_len);
 
 		cur += cur_len;
 	}