[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/drivers/s390/block/dasd.c b/drivers/s390/block/dasd.c
index bb72e0a..ac6d4d3 100644
--- a/drivers/s390/block/dasd.c
+++ b/drivers/s390/block/dasd.c
@@ -2299,9 +2299,8 @@
 	 * in the other openers.
 	 */
 	if (device->block) {
-		struct dasd_block *block = device->block;
-		max_count = block->bdev ? 0 : -1;
-		open_count = (int) atomic_read(&block->open_count);
+		max_count = device->block->bdev ? 0 : -1;
+		open_count = atomic_read(&device->block->open_count);
 		if (open_count > max_count) {
 			if (open_count > 0)
 				printk(KERN_WARNING "Can't offline dasd "