selftests/mm/kugepaged: Restore thp settings at exit

Previously, the saved thp settings would be restored upon a signal or at
the natural end of the test suite. But there are some tests that
directly call exit() upon failure. In this case, the thp settings were
not being restored, which could then influence other tests.

Fix this by installing an atexit() handler to do the actual restore. The
signal handler can now just call exit() and the atexit handler is
invoked.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>