[NET]: Revert skb_copy_datagram_iovec() recursion elimination.

Revert the following changeset:

bc8dfcb9



Recursive SKB frag lists are really possible and disallowing
them breaks things.

Noticed by: Jesse Brandeburg <jesse.brandeburg@intel.com>

Signed-off-by: David S. Miller <davem@davemloft.net>