sched: Introduce preempt_count accessor functions

Replace the single preempt_count() 'function' that's an lvalue with
two proper functions:

 preempt_count() - returns the preempt_count value as rvalue
 preempt_count_set() - Allows setting the preempt-count value

Also provide preempt_count_ptr() as a convenience wrapper to implement
all modifying operations.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-orxrbycjozopqfhb4dxdkdvb@git.kernel.org
[ Fixed build failure. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/init/main.c b/init/main.c
index af310af..7cc4b78 100644
--- a/init/main.c
+++ b/init/main.c
@@ -692,7 +692,7 @@
 
 	if (preempt_count() != count) {
 		sprintf(msgbuf, "preemption imbalance ");
-		preempt_count() = count;
+		preempt_count_set(count);
 	}
 	if (irqs_disabled()) {
 		strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf));