kselftest: rtnetlink.sh: add verbose flag

Uses a run_cmd helper function similar to other selftests to add
verbose functionality i.e. print executed commands and their outputs

Many commands silence or redirect output. This can be removed since
the verbose helper function captures output anyway and only outputs it
if VERBOSE is true. Similarly, the helper command for pipes to grep
searches stderr and stdout. This makes output redirection unnecessary
in those cases.
Signed-off-by: Daniel Mendes <dmendes@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>