tools/cephfs: set dir_layout when injecting inodes

tools/cephfs: set dir_layout when injecting inodes

When we left this as zero, the MDS would interpret it was HASH_LINUX
rather than the default HASH_RJENKINS.  Potentially that
could cause problems if there perhaps were already dirfrags in
the metadata pool that were set up using rjenkins.  Mainly
it just seems more appropriate to explicitly set this field
rather than hit the fallback behaviour.

Related: http://tracker.ceph.com/issues/19406
Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 7d6d542885bd29b71214f9ca52bd26e9183c5d01)