ch_ktls: missing handling of header alone

If an skb has only header part which doesn't start from
beginning, is not being handled properly.

Fixes: dc05f3df

 ("chcr: Handle first or middle part of record")
Signed-off-by: Rohit Maheshwari <rohitm@chelsio.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>