selftests/mm: give scripts execute permission

When run under run_vmtests.sh, test scripts were failing to run with
"permission denied" due to the scripts not being executable.

It is also annoying not to be able to directly invoke run_vmtests.sh,
which is solved by giving also it the execute permission.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ryan Roberts <[e-mail]>
Cc: SeongJae Park <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Florent Revest <[e-mail]>
Cc: Jérôme Glisse <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Liam R. Howlett <[e-mail]>
Cc: Mark Brown <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>