1, 3 and 4 are just mechanical fixes to make code cleaner.

2 fixes the bug that netfront generates oversize skbs. The fix was suggested by
Ben. My test shows that netfront doesn't generate oversize skb after this fix.

4 tries to coalesce slots before copying. This version enlarges the grant copy
buffer to avoid overflow.

6 loosens the punishment when netback sees oversize packet.


Wei.