net: tcp: prepare_segment needs to work with pre-allocated buffers

Transmitted data coming in via net_context_send() must already have an
allocated TX nbuf.  Let prepare_segment() work with one if available,
adding any trailing data to the end of the generated buf chain.

Change-Id: I4f084ce51e6713773a48b8c9c59450656bc87d8e
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>