selftests: mptcp: print extra msg in chk_csum_nr

When the multiple checksum errors occur in chk_csum_nr(), print the
numbers of the errors as an extra message.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>