backports: add GRO_* enums, and its new gro_result_t typedef

backports: add GRO_* enums, and its new gro_result_t typedef

This backports commit d1c76af9e added by Herbert which added the first
enums and then commit 5b252f0c2 added by Ben which gave the enums a
label and then pegged a typedef onto these. This backports these commits
in a forward compatible way upkeeping the introduction and then the
typedef. In order to make this work we use make backported code use
enums with the BACKPORT_prefix but upkeep the same values, this lets
us declare the typedef and peg it to a similar set.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>