memory hotplug: fix early allocation handling

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.

[[email protected]: coding-style fix]
Reviewed-by: Andy Whitcroft <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Acked-by: Yasunori Goto <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>