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 <[e-mail]>
Reviewed-by: Benjamin Poirier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/forwarding/lib.sh

···
 # Defines
 
 # Can be overridden by the configuration file.
-PING=${PING:=ping}
-PING6=${PING6:=ping6}
-MZ=${MZ:=mausezahn}
-MZ_DELAY=${MZ_DELAY:=0}
-ARPING=${ARPING:=arping}
-TEAMD=${TEAMD:=teamd}
-WAIT_TIME=${WAIT_TIME:=5}
-PAUSE_ON_FAIL=${PAUSE_ON_FAIL:=no}
-PAUSE_ON_CLEANUP=${PAUSE_ON_CLEANUP:=no}
-NETIF_TYPE=${NETIF_TYPE:=veth}
-NETIF_CREATE=${NETIF_CREATE:=yes}
-MCD=${MCD:=smcrouted}
-MC_CLI=${MC_CLI:=smcroutectl}
-PING_COUNT=${PING_COUNT:=10}
-PING_TIMEOUT=${PING_TIMEOUT:=5}
-WAIT_TIMEOUT=${WAIT_TIMEOUT:=20}
-INTERFACE_TIMEOUT=${INTERFACE_TIMEOUT:=600}
-LOW_AGEING_TIME=${LOW_AGEING_TIME:=1000}
-REQUIRE_JQ=${REQUIRE_JQ:=yes}
-REQUIRE_MZ=${REQUIRE_MZ:=yes}
-REQUIRE_MTOOLS=${REQUIRE_MTOOLS:=no}
-STABLE_MAC_ADDRS=${STABLE_MAC_ADDRS:=no}
-TCPDUMP_EXTRA_FLAGS=${TCPDUMP_EXTRA_FLAGS:=}
-TROUTE6=${TROUTE6:=traceroute6}
+: "${PING:=ping}"
+: "${PING6:=ping6}"
+: "${MZ:=mausezahn}"
+: "${MZ_DELAY:=0}"
+: "${ARPING:=arping}"
+: "${TEAMD:=teamd}"
+: "${WAIT_TIME:=5}"
+: "${PAUSE_ON_FAIL:=no}"
+: "${PAUSE_ON_CLEANUP:=no}"
+: "${NETIF_TYPE:=veth}"
+: "${NETIF_CREATE:=yes}"
+: "${MCD:=smcrouted}"
+: "${MC_CLI:=smcroutectl}"
+: "${PING_COUNT:=10}"
+: "${PING_TIMEOUT:=5}"
+: "${WAIT_TIMEOUT:=20}"
+: "${INTERFACE_TIMEOUT:=600}"
+: "${LOW_AGEING_TIME:=1000}"
+: "${REQUIRE_JQ:=yes}"
+: "${REQUIRE_MZ:=yes}"
+: "${REQUIRE_MTOOLS:=no}"
+: "${STABLE_MAC_ADDRS:=no}"
+: "${TCPDUMP_EXTRA_FLAGS:=}"
+: "${TROUTE6:=traceroute6}"
 
 net_forwarding_dir=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")
 

tools/testing/selftests/net/forwarding/tc_common.sh

···
 CHECK_TC="yes"
 
 # Can be overridden by the configuration file. See lib.sh
-TC_HIT_TIMEOUT=${TC_HIT_TIMEOUT:=1000} # ms
+: "${TC_HIT_TIMEOUT:=1000}" # ms
 
 tc_check_packets()
 {

tools/testing/selftests/net/lib.sh

···
 ##############################################################################
 # Defines
 
-WAIT_TIMEOUT=${WAIT_TIMEOUT:=20}
+: "${WAIT_TIMEOUT:=20}"
+
 BUSYWAIT_TIMEOUT=$((WAIT_TIMEOUT * 1000)) # ms
 
 # Kselftest framework requirement - SKIP code is 4.