perf trace: Pass the richer 'struct syscall_arg' pointer to trace__btf_scnprintf()

Since we'll need it later in the current patch series and we can get the
syscall_arg_fmt from syscall_arg->fmt.

Based-on-a-patch-by: Howard Chu <howardchu95@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/Zsd8vqCrTh5h69rp@x1


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