perf: Fix callchain hit bad cast on ascii display

ipchain__fprintf_graph() casts the number of hits in a branch as an
int, which means we lose its highests bits.

This results in meaningless number of callchain hits in perf.data
that have a high number of hits recorded, typically those that have
callchain branches hits appearing more than INT_MAX. This happens
easily as those are pondered by the event period.

Reported-by: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>