rhel: limit stack size to 2M.

rhel: limit stack size to 2M.

The default stack size in Fedora/RHEL is 8M, which means when ovs-vswitchd
daemon starts and uses --mlockall (default), it will dirty all memory
regions for all threads which is proportionally to the number of CPUs.

On a big host this increases memory usage to many hundreds of megabytes
while OVS actually requires much less.

This patch relies on systemd to limit to 2M/thread. That is much more
than the minimum documented at function ovs_thread_create():

    /* Some small systems use a default stack size as small as 80 kB, but OVS
     * requires approximately 384 kB according to the following analysis:
     * https://mail.openvswitch.org/pipermail/ovs-dev/2016-January/308592.html
     *
     * We use 512 kB to give us some margin of error. */

Acked-By: Timothy Redaelli <tredaelli@redhat.com>
Tested-By: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>