net: remove k{un}map_skb_frag()

Since commit 3e4d3af5 (mm: stack based kmap_atomic()) we dont have
to disable BH anymore while mapping skb frags.

We can remove kmap_skb_frag() / kunmap_skb_frag() helpers and use
kmap_atomic() / kunmap_atomic()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>