Static: Fix bug in static route filter expressions

During reconfiguration, old and new filter expressions in static routes
are compared using i_same() function. When filter expressions contain
function calls, it is necessary that old filter expressions are the
second argument in i_same(), as it is internally modified by i_same().
Otherwise pointers to old (and freed) data appear in the config
structure.

Thanks to Lennert Buytenhek for tracking and reporting the bug.