selftests/vm: fix va_128TBswitch.sh permissions

Restore the +x bit to va_128TBswitch.sh, which got dropped from the
previous patch, somehow.

Link: https://lkml.kernel.org/r/20220708090646.34927-1-adam@wowsignal.io
Fixes: 1afd01d43efc3 ("selftests/vm: Only run 128TBswitch with 5-level paging")
Signed-off-by: Adam Sindelar <adam@wowsignal.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>