net: ip: context: Merge send_data() into the only caller

sendto() is one and only caller of send_data(), a function of handful
of lines, and yet send_data() is located a hundred lines away from
it. Such "spaghetti functions" complicate review, debugging, and
refactoring of the IP stack.

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