find_mergeable_vma() can return NULL.  In this case, it leads to a crash
when we access vm_mm(its offset is 0x40) later in write_protect_page.
And this case did happen on our server.  The following call trace is
captured in kernel 4.19 with the following patch applied and KSM zero
page enabled on our server.

  commit e86c59b1 ("mm/ksm: improve deduplication of zero pages with colouring")

So add a vma check to fix it.

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
  Oops: 0000 [#1] SMP NOPTI
  CPU: 9 PID: 510 Comm: ksmd Kdump: loaded Tainted: G OE 4.19.36.bsk.9-amd64 #4.19.36.bsk.9
  RIP: try_to_merge_one_page+0xc7/0x760
  Code: 24 58 65 48 33 34 25 28 00 00 00 89 e8 0f 85 a3 06 00 00 48 83 c4
        60 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 8b 46 08 a8 01 75 b8 <49>
        8b 44 24 40 4c 8d 7c 24 20 b9 07 00 00 00 4c 89 e6 4c 89 ff 48
  RSP: 0018:ffffadbdd9fffdb0 EFLAGS: 00010246
  RAX: ffffda83ffd4be08 RBX: ffffda83ffd4be40 RCX: 0000002c6e800000
  RDX: 0000000000000000 RSI: ffffda83ffd4be40 RDI: 0000000000000000
  RBP: ffffa11939f02ec0 R08: 0000000094e1a447 R09: 00000000abe76577
  R10: 0000000000000962 R11: 0000000000004e6a R12: 0000000000000000
  R13: ffffda83b1e06380 R14: ffffa18f31f072c0 R15: ffffda83ffd4be40
  FS: 0000000000000000(0000) GS:ffffa0da43b80000(0000) knlGS:0000000000000000
  CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000040 CR3: 0000002c77c0a003 CR4: 00000000007626e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  PKRU: 55555554
  Call Trace:
    ksm_scan_thread+0x115e/0x1960
    kthread+0xf5/0x130
    ret_from_fork+0x1f/0x30

[songmuchun@bytedance.com: if the vma is out of date, just exit]
  Link: http://lkml.kernel.org/r/20200416025034.29780-1-songmuchun@bytedance.com
[akpm@linux-foundation.org: add the conventional braces, replace /** with /*]
Fixes: e86c59b1 ("mm/ksm: improve deduplication of zero pages with colouring")
Co-developed-by: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Cc: Markus Elfring <Markus.Elfring@web.de>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200416025034.29780-1-songmuchun@bytedance.com
Link: http://lkml.kernel.org/r/20200414132905.83819-1-songmuchun@bytedance.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>