selftests/powerpc: Fix error handling in FPU/VMX preemption tests

selftests/powerpc: Fix error handling in FPU/VMX preemption tests

[ Upstream commit 9dbd5927408c4a0707de73ae9dd9306b184e8fee ]

The FPU & VMX preemption tests do not check for errors returned by the
low-level asm routines, preempt_fpu() / preempt_vsx() respectively.
That means any register corruption detected by the asm routines does not
result in a test failure.

Fix it by returning the return value of the asm routines from the
pthread child routines.

Fixes: e5ab8be68e44 ("selftests/powerpc: Test preservation of FPU and VMX regs across preemption")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231128132748.1990179-1-mpe@ellerman.id.au
Signed-off-by: Sasha Levin <sashal@kernel.org>