econet: fix CVE-2010-3848

Don't declare variable sized array of iovecs on the stack since this
could cause stack overflow if msg->msgiovlen is large.  Instead, coalesce
the user-supplied data into a new buffer and use a single iovec for it.

Signed-off-by: Phil Blundell <philb@gnu.org>
Signed-off-by: David S. Miller <davem@davemloft.net>