iproute2: ip maddress: Check multiaddr length

iproute2: ip maddress: Check multiaddr length

ip maddress add|del takes a MAC address as argument, so insist on
getting a length of ETH_ALEN bytes. This makes sure the passed argument
is actually a MAC address and especially not an IPv4 address which
was previously accepted and silently taken as a MAC address.

While at it, do not print *argv in the error path as this has been
modified by ll_addr_a2n() and doesn't contain the full string anymore,
which can lead to misleading error messages.

Also while at it, replace the hardcoded buffer size with the actual
buffer size using sizeof().

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>