ARM: KVM: vgic: take distributor lock on sync_hwstate path

Now that the maintenance interrupt handling is actually out of the
handler itself, the code becomes quite racy as we can get preempted
while we process the state.

Wrapping this code around the distributor lock ensures that we're not
preempted and relatively race-free.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>