fbarray: fix attach deadlock

fbarray: fix attach deadlock

rte_fbarray_attach() currently locks its internal
spinlock, but never releases it. Secondary processes
won't even start if there is more than one fbarray
to be attached to - the second rte_fbarray_attach()
would be just stuck.

Fix it by releasing the lock at the end of
rte_fbarray_attach(). I believe this was the original
intention.

Fixes: 5b61c62cfd76 ("fbarray: add internal tailq for mapped areas")

Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>