mptcp: drop useless "err = 0" in subflow_destroy

mptcp: drop useless "err = 0" in subflow_destroy

Upon successful return, mptcp_pm_parse_addr() returns 0. There is no need
to set "err = 0" after this. So after mptcp_nl_find_ssk() returns, just
need to set "err = -ESRCH", then release and free msk socket if it returns
NULL.

Also, no need to define the variable "subflow" in subflow_destroy(), use
mptcp_subflow_ctx(ssk) directly.

This patch doesn't change the behaviour of the code, just refactoring.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241213-net-next-mptcp-pm-misc-cleanup-v1-7-ddb6d00109a8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>