net: move gso declarations and functions to their own files

[ Upstream commit d457a0e3

 ]

Move declarations into include/net/gso.h and code into net/core/gso.c

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230608191738.3947077-1-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 7938cd15

 ("net: gro: fix misuse of CB in udp socket lookup")
Signed-off-by: Sasha Levin <sashal@kernel.org>