staging/lustre/llite: don't add to page cache upon failure

Reading directory pages may fail on MDS, in this case client should
not cache a non-up-to-date directory page, because it will cause
a later read on the same page fail.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5461
Reviewed-on: http://review.whamcloud.com/11450
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>