[SCSI] update SG_ALL to avoid causing chaining

Since the sg chaining patches went in, our current value of 255 for
SG_ALL excites chaining on some drivers which cannot support it (and
would thus oops).  Redefine SG_ALL to mean no sg table size
preference, but use the single allocation (non chained) limit.  This
also helps for drivers that use it to size an internal table.

We'll do an opt in system later where truly chaining supporting
drivers can define their sg_tablesize to be anything up to
SCSI_MAX_SG_CHAIN_ELEMENTS.

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