This patch fixes one of Christroph's fixme comments in the SCSI midlayer.

The selection of the serial number and pid for commands was done
by a global variable and not SMP safe. The race was not very
serious because it was only used for error handling, but it's
still better to fix it.

I audited all the drivers and none seemed to use them
for anything interesting, so I just made it a per host counter
protected by host lock.

(in fact they could be probably removed because they only see
to be used as a flag in exception handling and for debugging.
The patch would be unfortunately quite big because a lot
of driver printks use them)

This should be slight faster on SMP too because the cache line of
the counter won't bounce around the machine.
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>