net: remove last caller of skb_tail_offset() and itself

net: remove last caller of skb_tail_offset() and itself

Similar to the following commits:

commit 7f2c8d0563f11f04bf7df (netpoll: fix position of network header)
commit 897f8ba8d0db33bd7a780 (pktgen: Fix position of ip and udp header)

using skb_tail_offset() seems not correct since the offset
is based on head pointer.

With the last caller removed, skb_tail_offset() can be killed
finally.

Cc: Thomas Graf <tgraf@suug.ch>
Cc: Daniel Borkmann <dborkmann@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>