cluster/dht: assign layout onto missing directories too

The current self-healing algorithm is ignoring missing directories
for assigning new layout. When lookup() is racing against mkdir()
or when self-healing a half-done mkdir(), the layout assignment split
must happen based on the final number of directories, and not the
currently existing number of directories (because we finish mkdir()
of missing directories before hash layout assignment).

Without this fix, concurrent mkdir() and lookup() will step on
each others feet, create a messed up layout on disk, and end up
with different in-memory layouts.

Once two clients have different in-memory layouts, creation of
subdirectory will not arbitrate on the same hashed subvolume and will
result in GFID mismatch of the sub-directory.

Change-Id: Ia47acad67c265060405984c822b4d37512b9dbb3
BUG: 907072
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5871
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>