tools/testing/selftests/vm/hmm-tests.c: fix build

hmm-tests.c:1607:42: error: 'HMM_DMIRROR_MIGRATE' undeclared (first use in this function); did you mean 'HMM_DMIRROR_WRITE'?

Fixes: f6c3e1ae

 ("mm/hmm: add a test for cross device private faults")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>