eal: fix leak on device event callback unregister
eal: fix leak on device event callback unregister

The event_cb->dev_name is not freed when freeing event_cb,
and this causes a memory leak.

Fixes: a753e53d517b ("eal: add device event monitor framework")
Cc: stable@dpdk.org

Signed-off-by: Yunjian Wang <9985a735d5d5f80ef648228b285228d6eb6eaf45@huawei.com>
Acked-by: Jeff Guo <62aa901f8680899d1e8b3d9b45cf4843c2976e1c@intel.com>
Acked-by: David Marchand <f28529695cfa9e3e1eb84e7072aa626af4abb18e@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk