LU-10337 mdt: Allow open of open orphans

LU-10337 mdt: Allow open of open orphans

Standard open by handle behavior allows opening of open
unlinked files files.  This currently only works in Lustre
if the file is already open on the same node, which is
insufficient.

When an open file is unlinked, we make it an orphan.
These files can be recognized by checking their open count
(mod_count).  It's enough to just make opening these files
possible, because the client cannot look them up to do an
open except when using a file handle.

Cray-bug-id: LUS-2626
Change-Id: Idd7898cefcf60b28c682e578774411e476216c9e
Signed-off-by: Patrick Farrell <paf@cray.com>
Reviewed-on: https://review.whamcloud.com/30405
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>