[POWERPC] Initialise spinlock in the DEBUG_PAGEALLOC code

Fixes:

BUG: spinlock bad magic on CPU#0, swapper/0
 lock: c00000000064ec30, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
Call Trace:
[c00000000062b980] [c00000000000f920] .show_stack+0x6c/0x1a0 (unreliable)
[c00000000062ba20] [c0000000001c2b40] .spin_bug+0xb0/0xd4
[c00000000062bab0] [c0000000001c2ed0] ._raw_spin_lock+0x44/0x184
[c00000000062bb50] [c0000000003a42b4] ._spin_lock+0x10/0x24
[c00000000062bbd0] [c00000000002b4dc] .kernel_map_pages+0x198/0x278
[c00000000062bc90] [c000000000079720] .free_hot_cold_page+0x124/0x418
[c00000000062bd70] [c000000000530278] .free_all_bootmem_core+0x14c/0x224
[c00000000062be50] [c00000000052a178] .mem_init+0x68/0x170
[c00000000062bee0] [c00000000051d874] .start_kernel+0x2a0/0x37c
[c00000000062bf90] [c0000000000084c8] .start_here_common+0x54/0x8c

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>