[S390] fix tlb flushing vs. concurrent /proc accesses

The tlb flushing code uses the mm_users field of the mm_struct to
decide if each page table entry needs to be flushed individually with
IPTE or if a global flush for the mm_struct is sufficient after all page
table updates have been done. The comment for mm_users says "How many
users with user space?" but the /proc code increases mm_users after it
found the process structure by pid without creating a new user process.
Which makes mm_users useless for the decision between the two tlb
flusing methods. The current code can be confused to not flush tlb
entries by a concurrent access to /proc files if e.g. a fork is in
progres. The solution for this problem is to make the tlb flushing
logic independent from the mm_users field.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/mmu.h

@@ -2,6 +2,8 @@
 #define __MMU_H
 
 typedef struct {
+	atomic_t attach_count;
+	unsigned int flush_mm;
 	spinlock_t list_lock;
 	struct list_head crst_list;
 	struct list_head pgtable_list;