lib: handle ENOENT errors when processes exit during PID scanning

There is a race window between readdir() and fopen() in line 125~131 of lib
function get_used_pids. A process could terminate after its directory entry
is read but before its status file is opened.

When fopen() is called, the status file may no longer exist, resulting in an
error like:

  tag=msgstress01__with_dmesg_entry stime=1723563200
  ...
  tst_test.c:1617: TINFO: Timeout per run is 0h 03m 30s
  tst_pid.c:128: TBROK: Failed to open FILE '/proc/73429/status' for reading: ENOENT (2)

To resolve this, the file_lines_scanf() function is modified to handle ENOENT
errors gracefully when strict mode is not enabled. If fopen() fails due to
ENOENT, the function now returns 1 to skip the missing file instead of treating
it as a critical error.

Reported-by: Paul Bunyan <pbunyan@redhat.com>
Analysed-by: Charles Mirabile <cmirabil@redhat.com>
Signed-off-by: Li Wang <liwang@redhat.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>

lib/safe_file_ops.c

@@ -182,6 +182,9 @@ int file_lines_scanf(const char *file, const int lineno,
 
 	fp = fopen(path, "r");
 	if (fp == NULL) {
+		if (strict == 0 && errno == ENOENT)
+			return 1;
+
 		tst_brkm_(file, lineno, TBROK | TERRNO, cleanup_fn,
 			"Failed to open FILE '%s' for reading", path);
 		return 1;