net: fix a sparse warning

net: fix a sparse warning

Fix a sparse warning introduced by Commit
0b5e8b8eeae40bae6ad7c7e91c97c3c0d0e57882 (net: Add Geneve tunneling
protocol driver) caught by kbuild test robot:

  # apt-get install sparse
  #   git checkout 0b5e8b8eeae40bae6ad7c7e91c97c3c0d0e57882
  #     make ARCH=x86_64 allmodconfig
  #       make C=1 CF=-D__CHECK_ENDIAN__
  #
  #
  #       sparse warnings: (new ones prefixed by >>)
  #
  #       >> net/ipv4/geneve.c:230:42: sparse: incorrect type in assignment (different base types)
  #          net/ipv4/geneve.c:230:42:    expected restricted __be32 [addressable] [assigned] [usertype] s_addr
  #             net/ipv4/geneve.c:230:42:    got unsigned long [unsigned] <noident>
  #

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>