[SCSI] iscsi_tcp: enable sg chaining

The previous patches converted iscsi_tcp to support sg chaining.
This patch sets the proper flags and sets sg_table size to
4096. This allows fs io to be capped at max_sectors, but passthrough
IO to be limited by some other part of the kernel.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>