LU-6027 mdd: Don't list "trusted.link" for orphans

Commit 09fe679 prevents mdd_xattr_get() from accessing XATTR_NAME_LINK
EAs of orphan objects but leaves mdd_xattr_list() unchanged.  This is
problematic for fgetxattr() calls on orphan objects when EA cache
refills are involved.  To respond to such a refill, mdt lists all of
the object's EA names and gets the value for each name.  In the case
of an orphan, XATTR_NAME_LINK EA will be listed, but corresponding
mdd_xattr_get() call will return -ENOENT, causing the fgetxattr() call
to fail eventually.  This patch updates mdd_xattr_list() to filter out
XATTR_NAME_LINK EAs for orphan objects as well, following commit
09fe679.

Change-Id: Ie59e4c9342056bacf57a82cf9bf77cfdedc99f6d
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/12884
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>