net/mlx4: Treat VFs fair when handling comm_channel_events

Handling comm_channel_event in mlx4_master_comm_channel uses a double
loop to determine which slaves have requested work. The search is
always started at lowest slave. This leads to unfairness; lower VFs
tends to be prioritized over higher VFs.

The patch uses find_next_bit to determine which slaves to handle.
Fairness is implemented by always starting at the next to the last
start.

An MPI program has been used to measure improvements. It runs 500
ibv_reg_mr, synchronizes with all other instances and then runs 500
ibv_dereg_mr.

The results running 500 processes, time reported is for running 500
calls:

ibv_reg_mr:
             Mod.   Org.
mlx4_1    403.356ms 424.674ms
mlx4_2    403.355ms 424.674ms
mlx4_3    403.354ms 424.674ms
mlx4_4    403.355ms 424.674ms
mlx4_5    403.357ms 424.677ms
mlx4_6    403.354ms 424.676ms
mlx4_7    403.357ms 424.675ms
mlx4_8    403.355ms 424.675ms

ibv_dereg_mr:
             Mod.   Org.
mlx4_1    116.408ms 142.818ms
mlx4_2    116.434ms 142.793ms
mlx4_3    116.488ms 143.247ms
mlx4_4    116.679ms 143.230ms
mlx4_5    112.017ms 107.204ms
mlx4_6    112.032ms 107.516ms
mlx4_7    112.083ms 184.195ms
mlx4_8    115.089ms 190.618ms

Suggested-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Hans Westgaard Ry <hans.westgaard.ry@oracle.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>