[TIPC]: Force linearization of non-linear sk_buffs

This patch allows TIPC to process incoming messages that are
stored in a fragmented sk_buff, by forcing the linearization
of any such messages it receives.

Note: This is an interim solution to allow TIPC to operate with
Ethernet devices that generate non-linear buffers (such as the
gianfar driver), until such time as the rest of TIPC is enhanced
to handle sk_buffs with multiple data areas.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>