sparc64: fix sun4v_build_irq NULL pointer dereference

sparc64: fix sun4v_build_irq NULL pointer dereference

sun4v_build_irq assume the given irq number is valid and use
it to get the handler pointer, the pointer is dereference
without being checked and cause kernel panic.

The cause of the invalid irq is that the tx/rx irq have never
been free during device removal. irq number end up exhausted during
continuous device add/removal test.

tx/rx irq is allocated during vio_device_probe() using irq_alloc()
and cookie_assign(). To free the tx/rx irq, cookie_unassign() and
irq_free() is called when the device is removed.

Orabug: 23082240

Signed-off-by: Thomas Tai <thomas.tai@oracle.com>
Reviewed-by: Chris Hyser <chris.hyser@oracle.com>
(cherry picked from commit 80043637b8fb1eabc16ab5947019f4dcdbb8c79f)
Signed-off-by: Allen Pais <allen.pais@oracle.com>