isdnloop: use strlcpy() instead of strcpy()

[ Upstream commit f9a23c84486ed350cce7bb1b2828abd1f6658796 ]

These strings come from a copy_from_user() and there is no way to be
sure they are NUL terminated.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>