'hash' stores not the flow hash, but the index of the GRO bucket
corresponding to it.
Change its name to 'bucket' to avoid confusion while reading lines
like '__set_bit(hash, &napi->gro_bitmask)'.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>