vxlan: move to its own directory

[ Upstream commit 67653936 ]

vxlan.c has grown too long. This patch moves
it to its own directory. subsequent patches add new
functionality in new files.

Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 94d166c5 ("vxlan: calculate correct header length for GPE")
Signed-off-by: Sasha Levin <sashal@kernel.org>