openvswitch: Rename GENEVE_TUN_OPTS() to TUN_METADATA_OPTS()

Also factors out Geneve validation code into a new separate function
validate_and_copy_geneve_opts().

A subsequent patch will introduce VXLAN options. Rename the existing
GENEVE_TUN_OPTS() to reflect its extended purpose of carrying generic
tunnel metadata options.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>