GFS2: Hash the negative dentry during inode lookup

Fix a regression introduced by:
6d4ade98 GFS2: Add atomic_open support
where an early return misses d_splice_alias() which had been
adding the negative dentry.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>