tcp: minor optimization in tcp_add_backlog()

[ Upstream commit d519f350967a60b85a574ad8aeac43f2b4384746 ]

If packet is going to be coalesced, sk_sndbuf/sk_rcvbuf values
are not used. Defer their access to the point we need them.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: ec00ed472bdb ("tcp: avoid premature drops in tcp_add_backlog()")
Signed-off-by: Sasha Levin <sashal@kernel.org>