net/mlx5e: Fix potential null pointer dereference

Calls to kzalloc() and kvzalloc() should be null-checked
in order to avoid any potential failures. In this case,
a potential null pointer dereference.

Fix this by adding null checks for _parse_attr_ and _flow_
right after allocation.

Addresses-Coverity-ID: 1497154 ("Dereference before null check")
Fixes: c620b772 ("net/mlx5: Refactor tc flow attributes structure")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>