selftests: fib_nexthops: Make ping timeout configurable

Commit 49bb39bd ("selftests: fib_nexthops: Make the test more robust")
increased the timeout of ping commands to 5 seconds, to make the test
more robust. Make the timeout configurable using '-w' argument to allow
user to change it depending on the system that runs the test. Some systems
suffer from slow forwarding performance, so they may need to change the
timeout.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220519070921.3559701-1-amcohen@nvidia.com


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