net/mlx5: Fix deletion of duplicate rules

When a rule is duplicated, the refcount of the rule is increased so only
the second deletion of the rule should cause destruction of the FTE.
Currently, the FTE will be destroyed in the first deletion of rule since
the modify_mask will be 0.
Fix it and call to destroy FTE only if all the rules (FTE's children)
have been removed.

Fixes: 718ce4d6

 ("net/mlx5: Consolidate update FTE for all removal changes")
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>