gro: cleanup

This patch updates codes as follows:
- change appropriate names for internal structures, variants and functions
- update comments and the content of the gro programmer guide for better

gro: cleanup

This patch updates codes as follows:
- change appropriate names for internal structures, variants and functions
- update comments and the content of the gro programmer guide for better
understanding
- remove needless check and redundant comments

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Junjie Chen <junjie.j.chen@intel.com>

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