perf_counter: Always schedule all software counters in

Software counters aren't subject to the limitations imposed by the
fixed number of hardware counter registers, so there is no reason not
to enable them all in __perf_counter_sched_in.  Previously we used to
break out of the loop when we got to a group that wouldn't fit on the
PMU; with this we continue through the list but only schedule in
software counters (or groups containing only software counters) from
there on.

Signed-off-by: Paul Mackerras <paulus@samba.org>