selftests: net: libs: Change variable fallback syntax

The current syntax of X=${X:=X} first evaluates the ${X:=Y} expression,
which either uses the existing value of $X if there is one, or uses the
value of "Y" as a fallback, and assigns it to X. The expression is then
replaced with the now-current value of $X. Assigning that value to X once
more is meaningless.

So avoid the outer X=... bit, and instead express the same idea though the
do-nothing ":" built-in as : "${X:=Y}". This also cleans up the block
nicely and makes it more readable.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Link: https://lore.kernel.org/r/1890ddc58420c2c0d5ba3154c87ecc6d9faf6947.1711464583.git.petrm@nvidia.com


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