KVM: arm64: selftests: Fix thread migration in arch_timer_edge_cases

KVM: arm64: selftests: Fix thread migration in arch_timer_edge_cases

arch_timer_edge_cases tries to migrate itself across host cpus. Before
the first test, it migrates to cpu 0 by setting up an affinity mask with
only bit 0 set. After that it looks for the next possible cpu in the
current affinity mask which still has only bit 0 set. So there is no
migration at all.

Fix this by reading the default mask at start and use this to find
the next cpu in each iteration.

Signed-off-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20250605103613.14544-3-sebott@redhat.com
Signed-off-by: Marc Zyngier <maz@kernel.org>