selftests/xsk: Fix double free

Fix a double free at exit of the test suite.

Fixes: a693ff3e ("selftests/xsk: Add support for executing tests on physical device")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20220929090133.7869-1-magnus.karlsson@gmail.com