net, treewide: define and use MAC_ADDR_STR_LEN

There are a few places in the tree which compute the length of the
string representation of a MAC address as 3 * ETH_ALEN - 1. Define a
constant for this and use it where relevant. No functionality changes
are expected.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@verge.net.au>
Link: https://patch.msgid.link/20250312-netconsole-v6-1-3437933e79b8@purestorage.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>