selftests/powerpc/ptrace: Make child errors more obvious

Use the FAIL_IF() macro so that errors in the child report a line
number, rather than just silently exiting.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-11-mpe@ellerman.id.au