isdnloop: Validate NUL-terminated strings from user.

[ Upstream commit 77bc6bed7121936bb2e019a8c336075f4c8eef62 ]

Return -EINVAL unless all of user-given strings are correctly
NUL-terminated.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>