mptcp: userspace pm: use a single point of exit

Like in all other functions in this file, a single point of exit is used
when extra operations are needed: unlock, decrement refcount, etc.

There is no functional change for the moment but it is better to do the
same here to make sure all cleanups are done in case of intermediate
errors.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>