LU-9803 tests: cast st_blksize for printf

LU-9803 tests: cast st_blksize for printf

Compilation with -Werror=format complains about this printf. Expects
unsigned long but st_blksize has type __blksize_t. Cast it to unsigned
long for printf

Test-Parameters: trivial
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I1eeb5613e485132de8f0bce08bd4d89887e52cf6
Reviewed-on: https://review.whamcloud.com/28262
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>