selftests/timers: Quiet warning due to lack of return check on brk

The posix_timers.c test has a loop that tries to keep it in
kernel space, repeatedly calling brk(). However, it doesn't
check the return value, which causes warnings.

This patch adds a err value which captures the return value
and modifies the test so it will quit if a failure occurs.

Cc: Shuah Khan <[e-mail]>
Cc: Prarit Bhargava <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Richard Cochran <[e-mail]>
Signed-off-by: John Stultz <[e-mail]>
Tested-by: Prarit Bhargava <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/timers/posix_timers.c

···
 static void kernel_loop(void)
 {
 	void *addr = sbrk(0);
+	int err = 0;
 
-	while (!done) {
-		brk(addr + 4096);
-		brk(addr);
+	while (!done && !err) {
+		err = brk(addr + 4096);
+		err |= brk(addr);
 	}
 }
 
···
 
 int main(int argc, char **argv)
 {
-	int err;
-
 	printf("Testing posix timers. False negative may happen on CPU execution \n");
 	printf("based timers if other threads run on the CPU...\n");