kni: fix vhost build with kernels 3.19 and 4.0

kni: fix vhost build with kernels 3.19 and 4.0

Due to commit c0371da6 in kernel 3.19, which removed msg_iov
and msg_iovlen from struct msghdr, DPDK would not build.
Also, functions memcpy_toiovecend and memcpy_fromiovecend
were removed in commits ba7438ae and 57dd8a07, being substituted by
copy_from_iter and copy_to_iter.

This patch makes use of struct iov_iter, which has references
to msg_iov and msg_iovln, and makes use of copy_from_iter
and copy_to_iter.

Reported-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>