posix-timers: Move sequence logic into struct k_itimer

The posix timer signal handling uses siginfo::si_sys_private for handling
the sequence counter check. That indirection is not longer required and the
sequence count value at signal queueing time can be stored in struct
k_itimer itself.

This removes the requirement of treating siginfo::si_sys_private special as
it's now always zero as the kernel does not touch it anymore.

Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/all/20241105064213.852619866@linutronix.de

include/uapi/asm-generic/siginfo.h

@@ -46,7 +46,7 @@ union __sifields {
 		__kernel_timer_t _tid;	/* timer id */
 		int _overrun;		/* overrun count */
 		sigval_t _sigval;	/* same as below */
-		int _sys_private;       /* not to be passed to user */
+		int _sys_private;       /* Not used by the kernel. Historic leftover. Always 0. */
 	} _timer;
 
 	/* POSIX.1b signals */