tcp: tcp_mark_head_lost is only valid for sack-tcp

[ Upstream commit 636ef28d6e4d174e424102466caf572b0406fb0e ]

so tcp_is_sack/reno checks are removed from tcp_mark_head_lost.

Signed-off-by: zhang kai <zhangkaiheb@126.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: a6458ab7fd4f ("UPSTREAM: tcp: fix DSACK undo in fast recovery to call tcp_try_to_open()")
Signed-off-by: Sasha Levin <sashal@kernel.org>