LU-7530 mdt: Do not leak identity when no nodemap is present

LU-7530 mdt: Do not leak identity when no nodemap is present

It looks like sometimes nodemap structure on the export is not there
due to a race in old_init_ucred_common.
Move the nodemap check to the start not to leak identity reference
in such a case.

The bug was introduced in commit 2aea469a3a6e214d from LU-7199

Also silence the warning as there's nothing sysadmins could do when
it happens.

Change-Id: I5329ccb16201a71a263eb586e3a486b26ff238db
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/17519
Tested-by: Jenkins
Reviewed-by: Kit Westneat <kit.westneat@gmail.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>