extentwalker: use default sizing of TREE_SEARCH_V2 buffers

Now that we can guess the size more or less automatically, there's
no need to make it unnecessarily large.

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

lib/extentwalker.cc

@@ -496,7 +496,7 @@ namespace crucible {
 	BtrfsExtentWalker::Vec
 	BtrfsExtentWalker::get_extent_map(off_t pos)
 	{
-		BtrfsIoctlSearchKey sk(65536);
+		BtrfsIoctlSearchKey sk;
 		if (!m_root_fd) {
 			m_root_fd = m_fd;
 		}