mptfusion: combine fw_event_work and its event_data

Tack the firmware reply event_data payload to the end of its
corresponding struct fw_event_work allocation.  Rework fw_event_work
allocation calculations to include the event_data size where
appropriate.

This clarifies the code a bit and avoids the following smatch warnings:

  drivers/message/fusion/mptsas.c:1003 mptsas_queue_device_delete()
    error: memcpy() 'fw_event->event_data' too small (29 vs 36)

  drivers/message/fusion/mptsas.c:1017 mptsas_queue_rescan() error: not
    allocating enough data 168 vs 160

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Sreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>