mptcp: pm: more precise error messages

mptcp: pm: more precise error messages

Some errors reported by the userspace PM were vague: "this or that is
invalid".

It is easier for the userspace to know which part is wrong, instead of
having to guess that.

While at it, in mptcp_userspace_pm_set_flags() move the parsing after
the check linked to the local attribute.

Reviewed-by: Geliang Tang <geliang@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>