mptcp: check userspace pm flags

Just like MPTCP_PM_ADDR_FLAG_SIGNAL flag is checked in userspace PM
announce mptcp_pm_nl_announce_doit(), PM flags should be checked in
mptcp_pm_nl_subflow_create_doit() too.

If MPTCP_PM_ADDR_FLAG_SUBFLOW flag is not set, there's no flags field
in the output of dump_addr. This looks a bit strange:

        id 10 flags  10.0.3.2

This patch uses mptcp_pm_parse_entry() instead of mptcp_pm_parse_addr()
to get the PM flags of the entry and check it. MPTCP_PM_ADDR_FLAG_SIGNAL
flag shouldn't be set here, and if MPTCP_PM_ADDR_FLAG_SUBFLOW flag is
missing from the netlink attribute, always set this flag.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>