btrfs: use clear_page where appropriate

There's a helper to clear whole page, with a arch-specific optimized
code. The replaced cases do not seem to be in performace critical code,
but we still might get some percent gain.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -355,7 +355,7 @@ static void io_ctl_map_page(struct btrfs_io_ctl *io_ctl, int clear)
 	io_ctl->orig = io_ctl->cur;
 	io_ctl->size = PAGE_SIZE;
 	if (clear)
-		memset(io_ctl->cur, 0, PAGE_SIZE);
+		clear_page(io_ctl->cur);
 }
 
 static void io_ctl_drop_pages(struct btrfs_io_ctl *io_ctl)

fs/btrfs/scrub.c

@@ -1643,7 +1643,7 @@ static int scrub_write_page_to_dev_replace(struct scrub_block *sblock,
 	if (spage->io_error) {
 		void *mapped_buffer = kmap_atomic(spage->page);
 
-		memset(mapped_buffer, 0, PAGE_SIZE);
+		clear_page(mapped_buffer);
 		flush_dcache_page(spage->page);
 		kunmap_atomic(mapped_buffer);
 	}