gro_cells: reduce number of synchronize_net() calls

After cited commit, gro_cells_destroy() became damn slow
on hosts with a lot of cores.

This is because we have one additional synchronize_net() per cpu as
stated in the changelog.

gro_cells_init() is setting NAPI_STATE_NO_BUSY_POLL, and this was enough
to not have one synchronize_net() call per netif_napi_del()

We can factorize all the synchronize_net() to a single one,
right before freeing per-cpu memory.

Fixes: 5198d545

 ("net: remove napi_hash_del() from driver-facing API")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20201124203822.1360107-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>