Documentation: Add note about iproute2 requirements for check-kmod
On many systems the check-kmod and check-kernel test suites have
many failures due to the lack of feature support in the older
iproute2 utility packages shipped with those systems. Add a
note indicating that it might be necessary to update the iproute2
utility package in order to fix those errors.
Signed-off-by: Greg Rose <gvrose8192@gmail.com> Signed-off-by: William Tu <u9012063@gmail.com>