mptcp: pm: make three pm wrappers static

mptcp: pm: make three pm wrappers static

Three netlink functions:

	mptcp_pm_nl_get_addr_doit()
	mptcp_pm_nl_get_addr_dumpit()
	mptcp_pm_nl_set_flags_doit()

are generic, implemented for each PM, in-kernel PM and userspace PM. It's
clearer to move them from pm_netlink.c to pm.c.

And the linked three path manager wrappers

	mptcp_pm_get_addr()
	mptcp_pm_dump_addr()
	mptcp_pm_set_flags()

can be changed as static functions, no need to export them in protocol.h.

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>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>