LU-7939 osd-ldiskfs: check IS_ERR() instead of NULL

osd_ldiskfs_find_entry() returns ERR_PTR() but checked for NULL.

Change-Id: I9bf6a7f331fd1e6c34c7ea2209944748e832891d
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/19197
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>