perf evsel: Move ignore_missing_thread() to fallback code

perf evsel: Move ignore_missing_thread() to fallback code

This patch moves ignore_missing_thread outside the perf_event_open loop.

Doing so, we need to move the retry_open flag a few places higher, with
minimal impact. Furthermore, thread need not be decreased since it won't
get increased by the for loop (since we're jumping back inside), but we
need to check that the nthreads decrease didn't put thread out of range.

The goal is to have fallbacks handled in one place only, since in the
future parallel code, these would be handled separately.

Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/4eca51443c786baaf6811b7cd8e73aafd97f7606.1629490974.git.rickyman7@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>