s390/irq: reduce size of external interrupt handler hash array

Change the hash algorithm a bit so it produces only values in the
range of 0..31.
This allows to reduce the size of the external interrupt handler hash
array even further while making sure that each of the known interrupt
sources keeps its unique hash with the slightly modified algorithm:

0x1004 --> 12
0x1201 --> 10
0x1202 --> 11
0x1406 --> 16
0x1407 --> 17
0x2401 --> 19
0x2603 --> 22
0x4000 --> 0

This also means that the entire array now fits into exactly one cache
line; so add a proper align statement as well.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/kernel/irq.c

@@ -196,7 +196,7 @@ asmlinkage void do_softirq(void)
  * ext_int_hash[index] is the list head for all external interrupts that hash
  * to this index.
  */
-static struct hlist_head ext_int_hash[256];
+static struct hlist_head ext_int_hash[32] ____cacheline_aligned;
 
 struct ext_int_info {
 	ext_int_handler_t handler;
@@ -210,7 +210,9 @@ static DEFINE_SPINLOCK(ext_int_hash_lock);
 
 static inline int ext_hash(u16 code)
 {
-	return (code + (code >> 9)) & 0xff;
+	BUILD_BUG_ON(!is_power_of_2(ARRAY_SIZE(ext_int_hash)));
+
+	return (code + (code >> 9)) & (ARRAY_SIZE(ext_int_hash) - 1);
 }
 
 int register_external_interrupt(u16 code, ext_int_handler_t handler)