vrf: handle CONFIG_IPV6 not set for vrf_add_mac_header_if_unset()

vrf: handle CONFIG_IPV6 not set for vrf_add_mac_header_if_unset()

The vrf_add_mac_header_if_unset() is defined within a conditional
compilation block which depends on the CONFIG_IPV6 macro.
However, the vrf_add_mac_header_if_unset() needs to be called also by IPv4
related code and when the CONFIG_IPV6 is not set, this function is missing.
As a consequence, the build process stops reporting the error:

 ERROR: implicit declaration of function 'vrf_add_mac_header_if_unset'

The problem is solved by *only* moving functions
vrf_add_mac_header_if_unset() and vrf_prepare_mac_header() out of the
conditional block.

Reported-by: kernel test robot <[email protected]>
Fixes: 0489390882202 ("vrf: add mac header for tunneled packets when sniffer is attached")
Signed-off-by: Andrea Mayer <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>