userfaultfd: check VM_MAYWRITE was set after verifying the uffd is registered

Calling UFFDIO_UNREGISTER on virtual ranges not yet registered in uffd
could trigger an harmless false positive WARN_ON.  Check the vma is
already registered before checking VM_MAYWRITE to shut off the false
positive warning.

Link: http://lkml.kernel.org/r/20181206212028.18726-2-aarcange@redhat.com
Cc: <stable@vger.kernel.org>
Fixes: 29ec9066

 ("userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by:  <syzbot+06c7092e7d71218a2c16@syzkaller.appspotmail.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>