select/cons_res - fix bug with Dragonfly and --switches count timeout

Given a configuration with TopologyParam including Dragonfly option, if a
job requested --switches count, the count timeout specified by either
the job request or max_switch_wait SchedulerParameters was not respected.
This was due to leaf_switch_count variable not being incremented in
_eval_nodes_dfly() function when needed, as we do in _eval_nodes_topo(),
the later being a execution path which already succeed to wait for the
switch count timeout.

Bug 4056