powerpc/xive/spapr: correct bitmap allocation size

In the Linux kernel, the following vulnerability has been resolved:

powerpc/xive/spapr: correct bitmap allocation size

kasan detects access beyond the end of the xibm->bitmap allocation:

BUG: KASAN: slab-out-of-bounds in _find_first_zero_bit+0x40/0x140
Read of size 8 at addr c00000001d1d0118 by task swapper/0/1

CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.19.0-rc2-00001-g90df023b36dd #28
Call Trace:
[c00000001d98f770] [c0000000012baab8] dump_stack_lvl+0xac/0x108 (unreliable)
[c00000001d98f7b0] [c00000000068faac] print_report+0x37c/0x710
[c00000001d98f880] [c0000000006902c0] kasan_report+0x110/0x354
[c00000001d98f950] [c000000000692324] __asan_load8+0xa4/0xe0
[c00000001d98f970] [c0000000011c6ed0] _find_first_zero_bit+0x40/0x140
[c00000001d98f9b0] [c0000000000dbfbc] xive_spapr_get_ipi+0xcc/0x260
[c00000001d98fa70] [c0000000000d6d28] xive_setup_cpu_ipi+0x1e8/0x450
[c00000001d98fb30] [c000000004032a20] pSeries_smp_probe+0x5c/0x118
[c00000001d98fb60] [c000000004018b44] smp_prepare_cpus+0x944/0x9ac
[c00000001d98fc90] [c000000004009f9c] kernel_init_freeable+0x2d4/0x640
[c00000001d98fd90] [c0000000000131e8] kernel_init+0x28/0x1d0
[c00000001d98fe10] [c00000000000cd54] ret_from_kernel_thread+0x5c/0x64

Allocated by task 0:
kasan_save_stack+0x34/0x70
__kasan_kmalloc+0xb4/0xf0
__kmalloc+0x268/0x540
xive_spapr_init+0x4d0/0x77c
pseries_init_irq+0x40/0x27c
init_IRQ+0x44/0x84
start_kernel+0x2a4/0x538
start_here_common+0x1c/0x20

The buggy address belongs to the object at c00000001d1d0118
which belongs to the cache kmalloc-8 of size 8
The buggy address is located 0 bytes inside of
8-byte region [c00000001d1d0118, c00000001d1d0120)

The buggy address belongs to the physical page:
page:c00c000000074740 refcount:1 mapcount:0 mapping:0000000000000000 index:0xc00000001d1d0558 pfn:0x1d1d
flags: 0x7ffff000000200(slab|node=0|zone=0|lastcpupid=0x7ffff)
raw: 007ffff000000200 c00000001d0003c8 c00000001d0003c8 c00000001d010480
raw: c00000001d1d0558 0000000001e1000a 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
c00000001d1d0000: fc 00 fc fc fc fc fc fc fc fc fc fc fc fc fc fc
c00000001d1d0080: fc fc 00 fc fc fc fc fc fc fc fc fc fc fc fc fc
>c00000001d1d0100: fc fc fc 02 fc fc fc fc fc fc fc fc fc fc fc fc
^
c00000001d1d0180: fc fc fc fc 04 fc fc fc fc fc fc fc fc fc fc fc
c00000001d1d0200: fc fc fc fc fc 04 fc fc fc fc fc fc fc fc fc fc

This happens because the allocation uses the wrong unit (bits) when it
should pass (BITS_TO_LONGS(count) * sizeof(long)) or equivalent. With small
numbers of bits, the allocated object can be smaller than sizeof(long),
which results in invalid accesses.

Use bitmap_zalloc() to allocate and initialize the irq bitmap, paired with
bitmap_free() for consistency.