net: tcp: Use net_buf pointer directly for out-of-order recv queue

Using net_pkt for TCP out-of-order recv queue was an overshot, as the
mechanism mostly used net_buf operations directly anyway. It can be
easily replaced with a direct net_buf pointer, so that it's not longer
needed to hog one net_pkt per TCP context anymore.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit aca511c91fce1ffb75ccb3c6119f79e3755b90d4)