LU-9369 lfs: make lfs find work correctly

LU-9369 lfs: make lfs find work correctly

This patch resets fp_lmd->lmd_lmm.lmm_magic in case that the
previous file's lum is mistakenly used for the next directory in
cb_find_init().
This patch also improves sanity.sh test_56s and test_56t to verify
the fix.

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I717ec809ef199be50dd1c4c7e6152ab9aa223f94
Reviewed-on: https://review.whamcloud.com/26914
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>