mptcp: userspace pm rename remove_err to out

mptcp: userspace pm rename remove_err to out

The value of 'err' will not be only '-EINVAL', but can be '0' in some
cases.

So it's better to rename the label 'remove_err' to 'out' to avoid
confusions.

Suggested-by: Matthieu Baerts <matttbe@kernel.org>
Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231128-send-net-next-2023107-v4-6-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>