inet: defines IPPROTO_* needed for module alias generation

Commit cfd280c9

 ("net: sync some IP headers with glibc") changed a set of
define's to an enum (with no explanation why) which introduced a bug
in module mip6 where aliases are generated using the IPPROTO_* defines;
mip6 doesn't load if require_module called with the aliases from
xfrm_get_type().

Reverting this change back to define's to fix the aliases.

modinfo mip6 (before this change)
alias:          xfrm-type-10-IPPROTO_DSTOPTS
alias:          xfrm-type-10-IPPROTO_ROUTING

modinfo mip6 (after this change)
alias:          xfrm-type-10-43
alias:          xfrm-type-10-60

Signed-off-by: Jan Moskyto Matejka <mq@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>