jiffies: Introduce USER_TICK_USEC and redefine TICK_USEC

Since the subsequent changes will need a TICK_USEC definition
analogous to TICK_NSEC, rename the existing TICK_USEC as
USER_TICK_USEC, update its users and redefine TICK_USEC
accordingly.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

kernel/time/ntp.c

@@ -31,7 +31,7 @@
 
 
 /* USER_HZ period (usecs): */
-unsigned long			tick_usec = TICK_USEC;
+unsigned long			tick_usec = USER_TICK_USEC;
 
 /* SHIFTED_HZ period (nsecs): */
 unsigned long			tick_nsec;