net/mlx5e: fix a potential double-free in fs_udp_create_groups

When kcalloc() for ft->g succeeds but kvzalloc() for in fails,
fs_udp_create_groups() will free ft->g. However, its caller
fs_udp_create_table() will free ft->g again through calling
mlx5e_destroy_flow_table(), which will lead to a double-free.
Fix this by setting ft->g to NULL in fs_udp_create_groups().

Fixes: 1c80bd68 ("net/mlx5e: Introduce Flow Steering UDP API")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>