dm-latency: ignore first time calculation in is_dm_target_io_latency_ok()

the latency is calculated by "delta = total - last_total", where
last_total is initialized to 0 as,
	static unsigned long last_total;
which means is very possible for "delta >= latency_warning_nr" to be
true, in sequence is_dm_target_io_latency_ok() returns incorrect '0'.

This patch adds another static int firsttime which is initialized to 1,
and set to 0 after first time is_dm_target_io_latency_ok() is called. By
this method, the firsttime (a.k.a firsttime != 0) return value of
is_dm_target_io_latency_ok() will be '1'. Then we avoid a bogus failure
might be returned by is_dm_target_io_latency_ok().