net: be more restrictive with ethernet address format

net: be more restrictive with ethernet address format

The current ether_unformat_addr code was based off of
BSD ether_aton. That version changed what was allowed
by the cmdline ether address parser.
For example, it allows dropping leading zeros.

Change the code to be more restrictive and only allow the fully
expanded standard formats.

Bugzilla ID: 324
Fixes: 596d31092d32 ("net: add function to convert string to ethernet address")

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Olivier Matz <olivier.matz@6wind.com>