memory hotplug: fix early allocation handling

Trying to add memory via add_memory() from within an initcall function
results in

bootmem alloc of 163840 bytes failed!
Kernel panic - not syncing: Out of memory

This is caused by zone_wait_table_init() which uses system_state to decide
if it should use the bootmem allocator or not.

When initcalls are handled the system_state is still SYSTEM_BOOTING but
the bootmem allocator doesn't work anymore.  So the allocation will fail.

To fix this use slab_is_available() instead as indicator like we do it
everywhere else.

[akpm@linux-foundation.org: coding-style fix]
Reviewed-by: NAndy Whitcroft <apw@shadowen.org>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: NYasunori Goto <y-goto@jp.fujitsu.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2807,7 +2807,7 @@ int zone_wait_table_init(struct zone *zone, unsigned long zone_size_pages)
 	alloc_size = zone->wait_table_hash_nr_entries
 					* sizeof(wait_queue_head_t);
 
- 	if (system_state == SYSTEM_BOOTING) {
+	if (!slab_is_available()) {
 		zone->wait_table = (wait_queue_head_t *)
 			alloc_bootmem_node(pgdat, alloc_size);
 	} else {