selftests/vm: 10x speedup for hmm-tests

selftests/vm: 10x speedup for hmm-tests

This patch reduces the running time for hmm-tests from about 10+ seconds,
to just under 1.0 second, for an approximately 10x speedup.  That brings
it in line with most of the other tests in selftests/vm, which mostly run
in < 1 sec.

This is done with a one-line change that simply reduces the number of
iterations of several tests, from 256, to 10.  Thanks to Ralph Campbell
for suggesting changing NTIMES as a way to get the speedup.

Suggested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: SeongJae Park <sj38.park@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Link: https://lkml.kernel.org/r/20201003011721.44238-1-jhubbard@nvidia.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>