selftests: mptcp: add more arguments for chk_join_nr

This patch added five more arguments for chk_join_nr(). The default
values of them are all zero.

The first two, csum_ns1 and csum_ns1, are passed to chk_csum_nr(), to
check the mib counters of the checksum errors in ns1 and ns2. A '+'
can be added into this two arguments to represent that multiple
checksum errors are allowed when doing this check. For example,

        chk_csum_nr "" +2 +2

indicates that two or more checksum errors are allowed in both ns1 and
ns2.

The remaining two, fail_nr and rst_nr, are passed to chk_fail_nr() and
chk_rst_nr() respectively, to check the sending and receiving mib
counters of MP_FAIL and MP_RST.

Also did some cleanups in chk_fail_nr(), renamed two local variables
and updated the output message.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>