LU-12125 mds: allow parallel directory rename

LU-12125 mds: allow parallel directory rename

Allow rename of subdirectories in the same parent directory to be
done in parallel, by only taking the DLM lock on the parent FID,
without locking the global LUSTRE_BFL_FID (Big Filesystem Lock).

There will still be proper serialization from the parent directory
FID lock for other rename operations affecting that directory or the
subdirectories themselves.  Since the subdirectories are known to be
within the same parent, there is no concern of "finding the parent"
to determine locking order.

The same compatibility rules apply as with parallel file renames.

We no longer need the target file type in this case, since the
source and target file type are verfied to be the same under lock.
We may again need a file type check for regular file renames across
different parent directories/shards, so it may still be useful.

Test-Parameters: testlist=racer env=DURATION=3600
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I9db8acb515f5274fa09f5a5f3d18504d4d3ebbe5