net/mlx5: Fix dereference on pointer attr after null check

When removing a flow from the slow path fdb, a flow attr struct is
allocated for the rule removal process. If the allocation fails the
code prints a warning message but continues with the removal flow
which include dereferencing a pointer which could be null.
Fix this by exiting the function in case the attr allocation failed.

Fixes: c620b772 ("net/mlx5: Refactor tc flow attributes structure")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ariel Levkovich <lariel@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>