crucible: use set instead of vector in BtrfsExtentWalker

This gets rid of some more big memsets.  It may replace them
with a lot of tiny mallocs, though.  If this turns out to be
a bad idea then at least we can easily revert the change.

lib/fs.cc

@@ -707,6 +707,12 @@ namespace crucible {
 		return offset + len;
 	}
 
+	bool
+	BtrfsIoctlSearchHeader::operator<(const BtrfsIoctlSearchHeader &that) const
+	{
+		return tie(objectid, type, offset, len, transid) < tie(that.objectid, that.type, that.offset, that.len, that.transid);
+	}
+
 	bool
 	BtrfsIoctlSearchKey::do_ioctl_nothrow(int fd)
 	{
@@ -727,13 +733,12 @@ namespace crucible {
 		static_cast<btrfs_ioctl_search_key&>(*this) = ioctl_ptr->key;
 
 		m_result.clear();
-		m_result.reserve(nr_items);
 
 		size_t offset = pointer_distance(ioctl_ptr->buf, ioctl_ptr);
 		for (decltype(nr_items) i = 0; i < nr_items; ++i) {
 			BtrfsIoctlSearchHeader item;
 			offset = item.set_data(ioctl_arg, offset);
-			m_result.push_back(item);
+			m_result.insert(item);
 		}
 
 		return true;