selftests/filesystems: grant executable permission to run_fat_tests.sh

When use tools/testing/selftests/kselftest_install.sh to make the
kselftest-list.txt under tools/testing/selftests/kselftest_install.

Then use tools/testing/selftests/kselftest_install/run_kselftest.sh to run
all the kselftests in kselftest-list.txt, it will be blocked by case
"filesystems/fat: run_fat_tests.sh" with "Warning: file run_fat_tests.sh
is not executable", so grant executable permission to run_fat_tests.sh to
fix this issue.

Link: https://lkml.kernel.org/r/dfdbba6df8a1ab34bb1e81cd8bd7ca3f9ed5c369.1673424747.git.pengfei.xu@intel.com
Fixes: dd7c9be3

 ("selftests/filesystems: add a vfat RENAME_EXCHANGE test")
Signed-off-by: Pengfei Xu <pengfei.xu@intel.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>