selftests: mptcp: join: specify host being checked

selftests: mptcp: join: specify host being checked

Instead of displaying 'invert' when looking at some events like MP_FAIL,
MP_FASTCLOSE, MP_RESET, RM_ADDR, which is a bit vague because they are
not traditionnaly sent from one side, the host being checked is now
printed.

For the ADD_ADDR, only display the host when it is the client sending
it, which is more unusual.

Also before, the 'invert' message was printed after a few checks, but it
was not clear which ones exactly.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240902-net-next-mptcp-mib-mpjtx-misc-v1-8-d3e0f3773b90@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>