perf trace: Streamline validation of select syscall names list

Rename the 'i' variable to 'nr_used' and use set 'nr_allocated' since
the start of this function, leaving the final assignment of the longer
named trace->ev_qualifier_ids.nr state to 'nr_used' at the end of the
function.

No change in behaviour intended.

Cc: Leo Yan <leo.yan@linaro.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lkml.kernel.org/n/tip-kpgyn8xjdjgt0timrrnniquv@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>