FROMLIST: KVM: arm64: vgic: Explicitly implement vgic_dist::ready ordering

In preparation to remove synchronize_srcu() from MMIO registration,
remove the distributor's dependency on this implicit barrier by
direct acquire-release synchronization on the flag write and its
lock-free check.

Bug: 395485007
Bug: 357781595
Link: https://lore.kernel.org/all/20250624092256.1105524-3-keirf@google.com/



Change-Id: Id23675e275d78a387d5cb884c4c7da9a5aefe7fe
Signed-off-by: Keir Fraser <keirf@google.com>