selftests/vm: 2x speedup for run_vmtests.sh

Each invocation of userfaultfd for "anon" and "shmem" was taking about
6.5 sec to run, contributing to an overall run time of about 22 sec for
run_vmtests.sh.

Reduce the size and bounce input values to the userfaultfd invocation
within run_vmtests.sh, enough to get each invocation down to about 1.0
sec. This should still provide a reasonable smoke test, while staying
within a nominal time budget of around 1 second or so per test. And this
brings the overall running time of run_vmtests.sh down to 11 second.

Link: https://lkml.kernel.org/r/20201026064021.3545418-10-jhubbard@nvidia.com


Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>