um,ethertap: use strncpy

[um maintainers appear to be vanished]

I can't prove the case pointed out in
https://bugzilla.kernel.org/show_bug.cgi?id=82341
is correct so let us play safe.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>