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/[email protected]
Fixes: 1afd01d43efc3 ("selftests/vm: Only run 128TBswitch with 5-level paging")

Signed-off-by: Adam Sindelar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: the80srobot