mm/device-public-memory: fix edge case in _vm_normal_page()

With device public pages at the end of my memory space, I'm getting
output from _vm_normal_page():

  BUG: Bad page map in process migrate_pages  pte:c0800001ffff0d06 pmd:f95d3000
  addr:00007fff89330000 vm_flags:00100073 anon_vma:c0000000fa899320 mapping:          (null) index:7fff8933
  file:          (null) fault:          (null) mmap:          (null) readpage:          (null)
  CPU: 0 PID: 13963 Comm: migrate_pages Tainted: P    B      OE 4.14.0-rc1-wip #155
  Call Trace:
     dump_stack+0xb0/0xf4 (unreliable)
     print_bad_pte+0x28c/0x340
     _vm_normal_page+0xc0/0x140
     zap_pte_range+0x664/0xc10
     unmap_page_range+0x318/0x670
     unmap_vmas+0x74/0xe0
     exit_mmap+0xe8/0x1f0
     mmput+0xac/0x1f0
     do_exit+0x348/0xcd0
     do_group_exit+0x5c/0xf0
     SyS_exit_group+0x1c/0x20
     system_call+0x58/0x6c

The pfn causing this is the very last one.  Correct the bounds check
accordingly.

Fixes: df6ad698

 ("mm/device-public-memory: device memory cache coherent with CPU")
Link: http://lkml.kernel.org/r/1506092178-20351-1-git-send-email-arbab@linux.vnet.ibm.com
Signed-off-by: Reza Arbab <arbab@linux.vnet.ibm.com>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>