ch_ktls: Correction in finding correct length

There is a possibility of linear skbs coming in. Correcting
the length extraction logic.

v2->v3:
- Separated un-related changes from this patch.

Fixes: 5a4b9fe7 ("cxgb4/chcr: complete record tx handling")
Signed-off-by: Rohit Maheshwari <rohitm@chelsio.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>