[SCSI] fix SLUB WARN_ON

We're getting a WARN_ON from SLUB indicating that we're trying to free
caches with in-use objects.  The root cause is a new dependency in the
command/sense free on unchecked_isa_dma.  The WARN_ON is caused by
drivers which change this in their setup after the command/sense cache
is allocated.

The fix is to move the allocation of this cache into scsi_add_host()
so things like gdth have an opportunity to modify it between alloc and
add (but *not* after).

The true fix would be to move unchecked_isa_dma into the template and
out of the host, so it because a truly read only variable.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>