net: tcp: Fix memory leak when lot of incoming packets

The code was leaking memory in TX side when there was lot of
incoming packets. The reason was that the net_pkt_sent() flag
was manipulated in two threads which caused races. The solution
is to move the sent flag check only to tcp.c.

Fixes #23246

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>