LU-5577 mdd: lu_dirent_calc_size() return type to size_t

LU-5577 mdd: lu_dirent_calc_size() return type to size_t

Change the type accordant usage.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I2451940225dd30015928cf85a2e0cc0e6cc8dfeb
Reviewed-on: http://review.whamcloud.com/12383
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>