[S390] Fix a lot of sparse warnings.

Most noteable part of this commit is the new local header file entry.h
which contains all the function declarations of functions that get only
called from asm code or are arch internal. That way we can avoid extern
declarations in C files.
This is more or less the same that was done for sparc64.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 5a445b1..0dfa988 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -44,6 +44,7 @@
 #include <asm/lowcore.h>
 #include <asm/sclp.h>
 #include <asm/cpu.h>
+#include "entry.h"
 
 /*
  * An array with a pointer the lowcore of every CPU.
@@ -297,7 +298,7 @@
 /*
  * this function sends a 'purge tlb' signal to another CPU.
  */
-void smp_ptlb_callback(void *info)
+static void smp_ptlb_callback(void *info)
 {
 	__tlb_flush_local();
 }