eal: fix config file path when checking process

eal: fix config file path when checking process

When primary process is booted with --file-prefix option, the API,
rte_eal_primary_proc_alive(), uses a wrong config file path to
check if primary process is alive.

Fix it by calling helper function to get config file path.

Fixes: dd3e00138d74 ("eal: check if primary process is alive")
Cc: stable@dpdk.org
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>