alarmtimer: Init nanosleep alarm timer on stack

syszbot reported the following debugobjects splat:

 ODEBUG: object is on stack, but not annotated
 WARNING: CPU: 0 PID: 4185 at lib/debugobjects.c:328

 RIP: 0010:debug_object_is_on_stack lib/debugobjects.c:327 [inline]
 debug_object_init+0x17/0x20 lib/debugobjects.c:391
 debug_hrtimer_init kernel/time/hrtimer.c:410 [inline]
 debug_init kernel/time/hrtimer.c:458 [inline]
 hrtimer_init+0x8c/0x410 kernel/time/hrtimer.c:1259
 alarm_init kernel/time/alarmtimer.c:339 [inline]
 alarm_timer_nsleep+0x164/0x4d0 kernel/time/alarmtimer.c:787
 SYSC_clock_nanosleep kernel/time/posix-timers.c:1226 [inline]
 SyS_clock_nanosleep+0x235/0x330 kernel/time/posix-timers.c:1204
 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

This happens because the hrtimer for the alarm nanosleep is on stack, but
the code does not use the proper debug objects initialization.

Split out the code for the allocated use cases and invoke
hrtimer_init_on_stack() for the nanosleep related functions.

Reported-by:  <syzbot+a3e0726462b2e346a31d@syzkaller.appspotmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: syzkaller-bugs@googlegroups.com
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1803261528270.1585@nanos.tec.linutronix.de