lacp: limit size of copied device name in ifr.ifname to IFNAMSIZ

This ensures there is at most as much data copied to the structure to be
used for the following ioctl as there is room for it.

strncpy will by itself fill with NULLs if the data to copy is shorter than
IFNAMSIZ.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>