s390: rename struct _lowcore to struct lowcore

Finally get rid of the leading underscore. I tried this already two or
three years ago, however Michael Holzheu objected since this would
break the crash utility (again).

However Michael integrated support for the new name into the crash
utility back then, so it doesn't break if the name will be changed
now.  So finally get rid of the ever confusing leading underscore.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/include/asm/vdso.h b/arch/s390/include/asm/vdso.h
index 787acd4..f9b02d3 100644
--- a/arch/s390/include/asm/vdso.h
+++ b/arch/s390/include/asm/vdso.h
@@ -42,8 +42,8 @@
 
 extern struct vdso_data *vdso_data;
 
-int vdso_alloc_per_cpu(struct _lowcore *lowcore);
-void vdso_free_per_cpu(struct _lowcore *lowcore);
+int vdso_alloc_per_cpu(struct lowcore *lowcore);
+void vdso_free_per_cpu(struct lowcore *lowcore);
 
 #endif /* __ASSEMBLY__ */
 #endif /* __S390_VDSO_H__ */