selftests: libs: Expand "$@" where possible

In some functions, argument-forwarding through "$@" without listing the
individual arguments explicitly is fundamental to the operation of a
function. E.g. xfail_on_veth() should be able to run various tests in the
fail-to-xfail regime, and usage of "$@" is appropriate as an abstraction
mechanism. For functions such as simple_if_init(), $@ is a handy way to
pass an array.

In other functions, it's merely a mechanism to save some typing, which
however ends up obscuring the real arguments and makes life hard for those
that end up reading the code.

This patch adds some of the implicit function arguments and correspondingly
expands $@'s. In several cases this will come in handy as following patches
adjust the parameter lists.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>