fs: don't zero-fill btrfs data containers

The kernel does it already, and we gain a little performance here because
we do it so often.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/fs.cc

@@ -188,7 +188,7 @@ namespace crucible {
 	}
 
 	BtrfsDataContainer::BtrfsDataContainer(size_t buf_size) :
-		m_data(buf_size, 0)
+		m_data(buf_size)
 	{
 	}