LU-9654 mdt: fix problem of RAoLU HSM policy

LU-9654 mdt: fix problem of RAoLU HSM policy

mdt_attr_get_complex() clears all known attributes even they are
already valid. So in mdt_handle_last_unlink(), the valid attributes
of HSM or nlink should be checked before calling that function.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I9ba561cadcc40baf5e28172cfda699cdecce7ea8
Reviewed-on: https://review.whamcloud.com/27564
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>