perf trace: Check and discard not only 'nr' but also '__syscall_nr'

Format fields of a syscall have the first variable '__syscall_nr' or
'nr' that mean the syscall number.  But it isn't relevant here so drop
it.

'nr' among fields of syscall was renamed '__syscall_nr'.  So add
exception handling to drop '__syscall_nr' and modify the comment for
this excpetion handling.

Reported-by: Arnaldo Carvalho de Melo <[email protected]>
Signed-off-by: Taeung Song <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Steven Rostedt <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: Taeung

tools/perf/builtin-trace.c

@@ -1725,8 +1725,12 @@ static int trace__read_syscall_info(struct trace *trace, int id)
 
 	sc->args = sc->tp_format->format.fields;
 	sc->nr_args = sc->tp_format->format.nr_fields;
-	/* drop nr field - not relevant here; does not exist on older kernels */
-	if (sc->args && strcmp(sc->args->name, "nr") == 0) {
+	/*
+	 * We need to check and discard the first variable '__syscall_nr'
+	 * or 'nr' that mean the syscall number. It is needless here.
+	 * So drop '__syscall_nr' or 'nr' field but does not exist on older kernels.
+	 */
+	if (sc->args && (!strcmp(sc->args->name, "__syscall_nr") || !strcmp(sc->args->name, "nr"))) {
 		sc->args = sc->args->next;
 		--sc->nr_args;
 	}