time: add function to convert between calendar time and broken-down time for universal use

There are many similar code in kernel for one object: convert time between
calendar time and broken-down time.

Here is some source I found:
  fs/ncpfs/dir.c
  fs/smbfs/proc.c
  fs/fat/misc.c
  fs/udf/udftime.c
  fs/cifs/netmisc.c
  net/netfilter/xt_time.c
  drivers/scsi/ips.c
  drivers/input/misc/hp_sdc_rtc.c
  drivers/rtc/rtc-lib.c
  arch/ia64/hp/sim/boot/fw-emu.c
  arch/m68k/mac/misc.c
  arch/powerpc/kernel/time.c
  arch/parisc/include/asm/rtc.h
  ...

We can make a common function for this type of conversion, At least we
can get following benefit:

1: Make kernel simple and unify
2: Easy to fix bug in converting code
3: Reduce clone of code in future
   For example, I'm trying to make ftrace display walltime,
   this patch will make me easy.

This code is based on code from glibc-2.6

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/time.h

@@ -155,6 +155,34 @@ extern void timekeeping_leap_insert(int leapsecond);
 struct tms;
 extern void do_sys_times(struct tms *);
 
+/*
+ * Similar to the struct tm in userspace <time.h>, but it needs to be here so
+ * that the kernel source is self contained.
+ */
+struct tm {
+	/*
+	 * the number of seconds after the minute, normally in the range
+	 * 0 to 59, but can be up to 60 to allow for leap seconds
+	 */
+	int tm_sec;
+	/* the number of minutes after the hour, in the range 0 to 59*/
+	int tm_min;
+	/* the number of hours past midnight, in the range 0 to 23 */
+	int tm_hour;
+	/* the day of the month, in the range 1 to 31 */
+	int tm_mday;
+	/* the number of months since January, in the range 0 to 11 */
+	int tm_mon;
+	/* the number of years since 1900 */
+	long tm_year;
+	/* the number of days since Sunday, in the range 0 to 6 */
+	int tm_wday;
+	/* the number of days since January 1, in the range 0 to 365 */
+	int tm_yday;
+};
+
+void time_to_tm(time_t totalsecs, int offset, struct tm *result);
+
 /**
  * timespec_to_ns - Convert timespec to nanoseconds
  * @ts:		pointer to the timespec variable to be converted