[SCSI] cxgb3i: transmit work-request fixes

- resize the work-request credit array to be based on skb's MAX_SKB_FRAGS.
- split the skb cb into tx and rx portion
- increase the default transmit window to 128K.
- stop queueing up the outgoing pdus if transmit window is full.

Signed-off-by: Karen Xie <kxie@chelsio.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>