macvlan: Replace strncpy() by strscpy()

The strncpy() function is being deprecated. Replace it by the safer
strscpy() and fix the following Coverity warning:

"Calling strncpy with a maximum size argument of 16 bytes on destination
array ifrr.ifr_ifrn.ifrn_name of size 16 bytes might leave the destination
string unterminated."

Notice that, unlike strncpy(), strscpy() always null-terminates the
destination string.

Addresses-Coverity-ID: 1445537 ("Buffer not null terminated")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>