mm/page_alloc.c: fix a crash in free_pages_prepare()

On architectures like s390, arch_free_page() could mark the page unused
(set_page_unused()) and any access later would trigger a kernel panic.
Fix it by moving arch_free_page() after all possible accessing calls.

 Hardware name: IBM 2964 N96 400 (z/VM 6.4.0)
 Krnl PSW : 0404e00180000000 0000000026c2b96e (__free_pages_ok+0x34e/0x5d8)
            R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
 Krnl GPRS: 0000000088d43af7 0000000000484000 000000000000007c 000000000000000f
            000003d080012100 000003d080013fc0 0000000000000000 0000000000100000
            00000000275cca48 0000000000000100 0000000000000008 000003d080010000
            00000000000001d0 000003d000000000 0000000026c2b78a 000000002717fdb0
 Krnl Code: 0000000026c2b95c: ec1100b30659 risbgn %r1,%r1,0,179,6
            0000000026c2b962: e32014000036 pfd 2,1024(%r1)
           #0000000026c2b968: d7ff10001000 xc 0(256,%r1),0(%r1)
           >0000000026c2b96e: 41101100  la %r1,256(%r1)
            0000000026c2b972: a737fff8  brctg %r3,26c2b962
            0000000026c2b976: d7ff10001000 xc 0(256,%r1),0(%r1)
            0000000026c2b97c: e31003400004 lg %r1,832
            0000000026c2b982: ebff1430016a asi 5168(%r1),-1
 Call Trace:
 __free_pages_ok+0x16a/0x5d8)
 memblock_free_all+0x206/0x290
 mem_init+0x58/0x120
 start_kernel+0x2b0/0x570
 startup_continue+0x6a/0xc0
 INFO: lockdep is turned off.
 Last Breaking-Event-Address:
 __free_pages_ok+0x372/0x5d8
 Kernel panic - not syncing: Fatal exception: panic_on_oops
 00: HCPGIR450W CP entered; disabled wait PSW 00020001 80000000 00000000 26A2379C

In the past, only kernel_poison_pages() would trigger this but it needs
"page_poison=on" kernel cmdline, and I suspect nobody tested that on
s390.  Recently, kernel_init_free_pages() (commit 6471384a ("mm:
security: introduce init_on_alloc=1 and init_on_free=1 boot options"))
was added and could trigger this as well.

[akpm@linux-foundation.org: add comment]
Link: http://lkml.kernel.org/r/1569613623-16820-1-git-send-email-cai@lca.pw


Fixes: 8823b1db ("mm/page_poison.c: enable PAGE_POISONING as a separate option")
Fixes: 6471384a ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options")
Signed-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Cc: <stable@vger.kernel.org>	[5.3+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>