selftests/x86/fsgsbase: Reap a forgotten child

The ptrace() test forgot to reap its child.  Reap it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/e7700a503f30e79ab35a63103938a19893dbeff2.1598461151.git.luto@kernel.org