eal: close multi-process socket during cleanup

eal: close multi-process socket during cleanup

When secondary process quit, the mp_socket* file still exist, that
cause rte_mp_request_sync fail when try to send message on a floating
socket.

The patch fix the issue by introduce a function rte_mp_channel_cleanup.
This function will be called by rte_eal_cleanup and it will close the
mp socket and delete the mp_socket* file.

Fixes: bacaa2754017 ("eal: add channel for multi-process communication")
Cc: stable@dpdk.org
Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>