net/mlx5e: fix double free of encap_header

When mlx5_packet_reformat_alloc() fails, the encap_header allocated in
mlx5e_tc_tun_create_header_ipv4{6} will be released within it. However,
e->encap_header is already set to the previously freed encap_header
before mlx5_packet_reformat_alloc(). As a result, the later
mlx5e_encap_put() will free e->encap_header again, causing a double free
issue.

mlx5e_encap_put()
    --> mlx5e_encap_dealloc()
        --> kfree(e->encap_header)

This happens when cmd: MLX5_CMD_OP_ALLOC_PACKET_REFORMAT_CONTEXT fail.

This patch fix it by not setting e->encap_header until
mlx5_packet_reformat_alloc() success.

Fixes: d589e785

 ("net/mlx5e: Allow concurrent creation of encap entries")
Reported-by: Cruz Zhao <cruzzhao@linux.alibaba.com>
Reported-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Dust Li <dust.li@linux.alibaba.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>