powerpc/selftests: Wait all threads to join

powerpc/selftests: Wait all threads to join

[ Upstream commit 693b31b2fc1636f0aa7af53136d3b49f6ad9ff39 ]

Test tm-tmspr might exit before all threads stop executing, because it just
waits for the very last thread to join before proceeding/exiting.

This patch makes sure that all threads that were created will join before
proceeding/exiting.

This patch also guarantees that the amount of threads being created is equal
to thread_num.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>