selftests: mptcp: join: reduce join_nr params

selftests: mptcp: join: reduce join_nr params

chk_join_nr() currently takes 9 positional parameters, 6 of them are
optional. It makes it hard to read:

  chk_join_nr 1 1 1 1 0 1 1 0 4

Naming these vars helps to make it easier to read:

  join_csum_ns1=1 join_csum_ns2=0 \
    join_fail_nr=1 join_rst_nr=1 join_infi_nr=0 \
    join_corrupted_pkts=4 \
    chk_join_nr 1 1 1

It will then be easier to add new optional parameters.

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-v1-4-d3e0f3773b90@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>