sunrpc: fixed rollback in rpc_gssd_dummy_populate()

__rpc_depopulate(gssd_dentry) was lost on error path

cc: stable@vger.kernel.org
Fixes: commit 4b9a445e ("sunrpc: create a new dummy pipe for gssd to hold open")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>