posix-timers: Make posix_clocks immutable

There are no more modular users providing a posix clock. The register
function is now pointless so the posix clock array can be initialized
statically at compile time and the array including the various k_clock
structs can be marked 'const'.

Inspired by changes in the Grsecurity patch set, but done proper.

[ tglx: Massaged changelog and fixed the POSIX_TIMER=n case ]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mike Travis <mike.travis@hpe.com>
Cc: Dimitri Sivanich <sivanich@hpe.com>
Link: http://lkml.kernel.org/r/20170526090311.3377-3-hch@lst.de
diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 8c1e43a..b313ef2 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -105,10 +105,11 @@ struct k_clock {
 			   struct itimerspec64 *cur_setting);
 };
 
-extern struct k_clock clock_posix_cpu;
-extern struct k_clock clock_posix_dynamic;
-
-void posix_timers_register_clock(const clockid_t clock_id, struct k_clock *new_clock);
+extern const struct k_clock clock_posix_cpu;
+extern const struct k_clock clock_posix_dynamic;
+extern const struct k_clock clock_process;
+extern const struct k_clock clock_thread;
+extern const struct k_clock alarm_clock;
 
 /* function to call to trigger timer event */
 int posix_timer_event(struct k_itimer *timr, int si_private);