LU-3915 osd-ldiskfs: don't assert on possible upgrade

LU-3915 osd-ldiskfs: don't assert on possible upgrade

There are some cases possible during upgrade to the Lustre 2.5
from older versions which need workaround instead assertion.
The osd_object_ref_del() shouldn't assert on nlink == 0 if
object is local file but produce error message.

The underlying ext4_unlink() code doesn't treat decref on an
inode with nlink == 0 as a fatal error either, so only print
an error in the non-upgrade case for regular objects.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I5aa1cc3d9a6d6210e8ce9f55193a4f25507b0986
Reviewed-on: http://review.whamcloud.com/7673
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>