exportfs: do not warn if /etc/exports.d/ does not exist

It isn't uncommon for /etc/exports.d/ to not exist, and imo, it's not
that big of a deal as often times, a simple /etc/exports is sufficient.
So silently skip the case where the dir is missing.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Steve Dickson <steved@redhat.com>