Even with the latest fixes aic79xx still occasionally triggers the
BUG_ON in slave_destroy. Rather than trying to figure out the various
levels of interaction here I've decided to remove the callback altogether.

The primary reason for the slave_alloc / slave_destroy is to keep an
index of pointers to the sdevs associated with a given target.
However, by changing the arguments to the affected functions slightly
it's possible to avoid the use of that index entirely.
The only performance penalty we'll incur is in writing the
information for /proc/scsi/XXX, as we'll have to recurse over all
available sdevs to find the correct ones. But I doubt that reading
from /proc is in any way time-critical.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>