LU-5937 lfs: ensure a valid directory size in lfs find

For a striped directory (and a striped file) the size returned by
LL_IOC_MDC_GETINFO may not be vaild. In cb_find_init() if the size of
a directory is needed then get it by calling fstat().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Di Wang <di.wang@intel.com>
Change-Id: Iddb9aa8e6664a09ff866a3995741cae17e1c9962
Reviewed-on: http://review.whamcloud.com/13456
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>