mem: fix cleanup after incomplete initialization

mem: fix cleanup after incomplete initialization

In case of EAL initialization failure rte_eal_memory_detach() may be
called before mapping memory configuration, which in this case points
to the static structure. Attempt to unmap it yields error:

    EAL: Could not unmap shared memory config: Invalid argument

Skip unmapping memory configuration if it's not yet shared.

Fixes: dfbc61a2f9a6 ("mem: detach memsegs on cleanup")

Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>