selftests/powerpc: Add test of fork with mapping above 512TB

[ Upstream commit 16391bfc

 ]

This tests that when a process with a mapping above 512TB forks we
correctly separate the parent and child address spaces. This exercises
the bug in the context id handling fixed in the previous commit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>