ip maddr: fix filtering by device

ip maddr: fix filtering by device

Commit 530903dd9003 ("ip: fix igmp parsing when iface is long") uses
variable len to keep trailing colon from interface name comparison.  This
variable is local to loop body but we set it in one pass and use it in
following one(s) so that we are actually using (pseudo)random length for
comparison. This became apparent since commit b48a1161f5f9 ("ipmaddr: Avoid
accessing uninitialized data") always initializes len to zero so that the
name comparison is always true. As a result, "ip maddr show dev eth0" shows
IPv4 multicast addresses for all interfaces.

Instead of keeping the length, let's simply replace the trailing colon with
a null byte. The bonus is that we get correct interface name in ma.name.

Fixes: 530903dd9003 ("ip: fix igmp parsing when iface is long")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Acked-by: Phil Sutter <phil@nwl.cc>
Acked-by: Petr Vorel <pvorel@suse.cz>