open: replace getdtablesize with getrlimit

The test currently uses getdtablesize() to determine the maximum
number of file descriptors for the process. This interface is
considered legacy and is not specified by POSIX.

Use getrlimit() instead, which provides a well-defined
and portable way to obtain the per-process file descriptor limit.

Link: https://lore.kernel.org/ltp/20260219141532.6513-2-always.starving0@gmail.com/
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Jinseok Kim <always.starving0@gmail.com>

Author: JawsKim

testcases/kernel/syscalls/open/open04.c

@@ -22,8 +22,10 @@ static char fname[20];
 static void setup(void)
 {
 	int fd;
+	struct rlimit rlim;
 
-	fds_limit = getdtablesize();
+	SAFE_GETRLIMIT(RLIMIT_NOFILE, &rlim);
+	fds_limit = rlim.rlim_cur;
 	first = SAFE_OPEN(FNAME, O_RDWR | O_CREAT, 0777);
 
 	fds = SAFE_MALLOC(sizeof(int) * (fds_limit - first));