mlx4_core: Fix GFP flags parameters to be gfp_t

Otherwise sparse gives a bunch of warnings like

    drivers/net/ethernet/mellanox/mlx4/srq.c:110:66: sparse: incorrect type in argument 4 (different base types)
    drivers/net/ethernet/mellanox/mlx4/srq.c:110:66:    expected int [signed] gfp
    drivers/net/ethernet/mellanox/mlx4/srq.c:110:66:    got restricted gfp_t

Signed-off-by: Roland Dreier <roland@purestorage.com>