selftests: forwarding: devlink_lib: Simplify deduction of DEVLINK_DEV

Use devlink -j and jq for more accurate querying. Use cut -f-2 instead
of rev-cut-rev combo.

Signed-off-by: Petr Machata <[e-mail]>
Signed-off-by: Ido Schimmel <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

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

···
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 # Defines
 
-DEVLINK_DEV=$(devlink port show | grep "${NETIFS[p1]}" | \
-	      grep -v "${NETIFS[p1]}[0-9]" | cut -d" " -f1 | \
-	      rev | cut -d"/" -f2- | rev)
+DEVLINK_DEV=$(devlink port show "${NETIFS[p1]}" -j \
+		     | jq -r '.port | keys[]' | cut -d/ -f-2)
 if [ -z "$DEVLINK_DEV" ]; then
 	echo "SKIP: ${NETIFS[p1]} has no devlink device registered for it"
 	exit 1