btrfs: drop pointless memset when cloning extent buffer

At btrfs_clone_extent_buffer(), before allocating the pages array for the
new extent buffer we are calling memset() to zero out the pages array of
the extent buffer. This is pointless however, because the extent buffer
already has every element in its pages array pointing to NULL, as it was
allocated with kmem_cache_zalloc(). The memset() was introduced with
commit dd137dd1 ("btrfs: factor out allocating an array of pages"),
but even before that commit we already depended on the pages array being
initialized to NULL for the error paths that need to call
btrfs_release_extent_buffer().

So remove the memset(), it's useless and slightly increases the object
text size.

Before this change:

   $ size fs/btrfs/extent_io.o
      text	   data	    bss	    dec	    hex	filename
     70580	   5469	     40	  76089	  12939	fs/btrfs/extent_io.o

After this change:

   $ size fs/btrfs/extent_io.o
      text	   data	    bss	    dec	    hex	filename
     70564	   5469	     40	  76073	  12929	fs/btrfs/extent_io.o
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4299,7 +4299,6 @@ struct extent_buffer *btrfs_clone_extent_buffer(const struct extent_buffer *src)
 	 */
 	set_bit(EXTENT_BUFFER_UNMAPPED, &new->bflags);
 
-	memset(new->pages, 0, sizeof(*new->pages) * num_pages);
 	ret = btrfs_alloc_page_array(num_pages, new->pages);
 	if (ret) {
 		btrfs_release_extent_buffer(new);