Nest: Avoid unnecessary net_format() in 'show route' command

When output of 'show route' command was generated, the net_format() was
called for each network prematurely, even if the result was not needed.

Fix the code to call net_format() only when needed. This makes queries
that process many networks but show only few (e.g. 'show route where ..',
or 'show route count') much faster (like 5x - 10x faster).