ceph: enable async dirops by default

Async dirops have been supported in mainline kernels for quite some time
now, and we've recently (as of June) started doing regular testing in
teuthology with '-o nowsync'. There were a few issues, but we've sorted
those out now.

Enable async dirops by default, and change /proc/mounts to show "wsync"
when they are disabled rather than "nowsync" when they are enabled.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>