net: tcp: prepare_segment: Return detailed error codes.

prepare_segment() returned NULL in case of any error, which then
net_context_send() translated into -EINVAL. That's highly confusing
though, because a common case of failure for prepare_segment() is
being unable to allocate data fragment(s) (for TCP header, etc.)
So, return output pkt by reference, and detailed error status as
a return value.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>