arm: vgic: fix race in vgic_vcpu_inject_irq

arm: vgic: fix race in vgic_vcpu_inject_irq

The initial check for a still pending interrupt (!list_empty(&n->inflight))
needs to be covered by the vgic lock to avoid trying to insert the IRQ into the
inflight list simultaneously on 2 pCPUS. Expand the area covered by the lock
appropriately.

Also consolidate the unlocks on the exit path into one location.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>