net/mlx4: fix drop flow resources leak

net/mlx4: fix drop flow resources leak

Resources allocated for drop flow rules are not freed properly. This causes
a memory leak and triggers an assertion failure on a reference counter when
compiled in debug mode.

This issue can be reproduced with testpmd by entering the following
commands:

 flow create 0 ingress pattern eth / end actions drop / end
 port start all
 port stop all
 port start all
 port stop all
 quit

The reason is additional references are taken when re-enabling existing
flow rules, a common occurrence when rehashing configuration.

Fixes: d3a7e09234e4 ("net/mlx4: allocate drop flow resources on demand")
Cc: stable@dpdk.org
Reported-by: Moti Haimovsky <motih@mellanox.com>
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>