selftests: mptcp: test userspace pm out of transfer

This patch moves userspace pm tests out of do_transfer(). Move add address
test into a new function userspace_pm_add_addr(), and remove address test
into userspace_pm_rm_sf_addr_ns1(). Move add subflow test into
userspace_pm_add_sf() and remove subflow into
userspace_pm_rm_sf_addr_ns2().

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230623-send-net-next-20230623-v1-1-a883213c8ba9@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>