perf_counter: Sleep before refresh using poll in perf top

perf top is refreshed every delay_secs the thread runs in such
loop:

while (sleep(delay_secs)) {
	print_sym_table();
}

At the end of print_sym_table(), poll is used without sleep delay
to check if we have something from stdin.

It means that this check is done only every delay_secs, which can
be higher that 2 secs if the user defined a custom refresh rate.

We can drop sleep() here and directly use poll to wait between
refresh periods, so that the reaction after the user stops perf top
after typing "Enter" is immediate and doesn't suffer from the
delay_secs latency.

Nb: poll doesn't add any overhead that can parasite perf top measures
since it sleeps the entire timeout here.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1244141284-7507-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>