test/mbuf: fix access to freed memory

test/mbuf: fix access to freed memory

Seen by ASan.

In the external buffer mbuf test, we check that the buffer is freed
by checking that its refcount is 0. This is not a valid condition,
because it accesses to an already freed area.

Fix this by setting a boolean flag in the callback when rte_free()
is actually called, and check this flag instead.

Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>