vxlan: fix sparse warnings

Sparse reports false positives for the header manipulation inlines. Annotate
them correctly.

Tested by sparse on a little endian and big endian machine.

Fixes: 54bfd872 ("vxlan: keep flags and vni in network byte order")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>