ARM/vgic: Fix out-of-bounds accesses in vgic_mmio_write_sgir()

ARM/vgic: Fix out-of-bounds accesses in vgic_mmio_write_sgir()

The switch() statement is over bits 24:25 (unshifted) of the guest provided
value.  This makes case 0x3: dead, and not an implementation of the 4th
possible state.

A guest which writes (0x3 << 24) | (0xff << 16) to this register will skip the
early exit, then enter bitmap_for_each() with targets not bound by nr_vcpus.

If the guest has fewer than 8 vCPUs, bitmap_for_each() will read off the end
of d->vcpu[] and use the resulting vcpu pointer to ultimately derive irq, and
perform out-of-bounds writes.

Fix this by changing case 0x3 to default.

Fixes: 08c688ca6422 ("ARM: new VGIC: Add SGIR register handler")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>