rtc: add the support for alarm time relative to current time in sysfs

In current kernel if we want to set the alarm time, the absolute time the
seconds relative to 1970-01-01 00:00:00) should be written into
/sys/class/rtc/rtc0/wakealarm. It is not convenient.

It is more reasonable to add the support for the alarm time relative to
current RTC time.(the unit is second)

For example:
If the RTC is required to generate alarm after 2 minutes, the following
will be OK.
echo +120 > /sys/class/rtc/rtc0/wakealarm
or echo +0x78 > /sys/class/rtc/rtc0/wakealarm

Signed-off-by: Zhao Yakui <[e-mail]>
Signed-off-by: Zhang Rui <[e-mail]>
Signed-off-by: David Brownell <[e-mail]>
Cc: Alessandro Zummo <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/rtc/rtc-sysfs.c

···
 	unsigned long now, alarm;
 	struct rtc_wkalrm alm;
 	struct rtc_device *rtc = to_rtc_device(dev);
+	char *buf_ptr;
+	int adjust = 0;
 
 	/* Only request alarms that trigger in the future.  Disable them
 	 * by writing another time, e.g. 0 meaning Jan 1 1970 UTC.
···
 		return retval;
 	rtc_tm_to_time(&alm.time, &now);
 
-	alarm = simple_strtoul(buf, NULL, 0);
+	buf_ptr = (char *)buf;
+	if (*buf_ptr == '+') {
+		buf_ptr++;
+		adjust = 1;
+	}
+	alarm = simple_strtoul(buf_ptr, NULL, 0);
+	if (adjust) {
+		alarm += now;
+	}
 	if (alarm > now) {
 		/* Avoid accidentally clobbering active alarms; we can't
 		 * entirely prevent that here, without even the minimal