Kernel crashes when a network module tries to copy a datagram to iovec in soft IRQ with paged data.
The problem is that kunmap function can't be called in IRQ.
All calls to kmap/kunmap fonctions are replaced by calls to kmap_skb_frag/kunmap_skb_frag when paged data are involved.

 <<datagram.c.patch>> 
Signed-off-by: Luc GORIA <luc.goria@adept-telecom.fr>