LU-3635 hsm: Fix ASSERT(inode->i_nlink > 0)

Two different handlers were setup to access Agent LLOG.
One for coordinator access, and one for /proc accesses.
Both tried to clean the LLOG content when closing it, leading to
crashes.
Both now used handler initialized by MDS.

Signed-off-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Change-Id: Ia6f22e9173266faf56a25e77fa5c9ae1bf69dc71
Reviewed-on: http://review.whamcloud.com/7111
Tested-by: Hudson
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>