perf jvmti: Address gcc string overflow warning for strncpy()

We are getting false positive gcc warning when we compile with gcc9 (9.1.1):

     CC       jvmti/libjvmti.o
   In file included from /usr/include/string.h:494,
                    from jvmti/libjvmti.c:5:
   In function ‘strncpy’,
       inlined from ‘copy_class_filename.constprop’ at jvmti/libjvmti.c:166:3:
   /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound depends on the length of the source argument [-Werror=stringop-overflow=]
     106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
         |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   jvmti/libjvmti.c: In function ‘copy_class_filename.constprop’:
   jvmti/libjvmti.c:165:26: note: length computed here
     165 |   size_t file_name_len = strlen(file_name);
         |                          ^~~~~~~~~~~~~~~~~
   cc1: all warnings being treated as errors

As per Arnaldo's suggestion use strlcpy(), which does the same thing and keeps
gcc silent.
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ben Gainey <ben.gainey@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20190531131321.GB1281@kravaSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/jvmti/libjvmti.c

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/compiler.h>
+#include <linux/string.h>
 #include <sys/types.h>
 #include <stdio.h>
 #include <string.h>
@@ -162,8 +163,7 @@ copy_class_filename(const char * class_sign, const char * file_name, char * resu
 		result[i] = '\0';
 	} else {
 		/* fallback case */
-		size_t file_name_len = strlen(file_name);
-		strncpy(result, file_name, file_name_len < max_length ? file_name_len : max_length);
+		strlcpy(result, file_name, max_length);
 	}
 }