caif: add a sanity check to the tty name

"tty->name" and "name" are a 64 character buffers.  My static checker
complains because we add the "cf" on the front so it look like we are
copying a 66 character string into a 64 character buffer.

Also if the name is larger than IFNAMSIZ (16) it triggers a BUG_ON()
inside the call to alloc_netdev().

This is all under CAP_SYS_ADMIN so it's not a security fix, it just adds
a little robustness.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>