utils: Introduce and use get_ifname_rta()

utils: Introduce and use get_ifname_rta()

Be consistent in handling of IFLA_IFNAME attribute in all places: if
there is no attribute report bug to stderr and use ll_idx_n2a() as
last measure to get name in "if%u" format instead of "<nil>".

Use check_ifname() to validate network device name: this catches both
unexpected return from kernel and ll_idx_n2a().

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>