LU-7988 hsm: remove hsd.request_sz from struct hsm_scan_data

LU-7988 hsm: remove hsd.request_sz from struct hsm_scan_data

request_sz is only used in one function, so it doesn't need to be part
of the structure. Instead make it local to mdt_coordinator().

Test-Parameters: trivial testlist=sanity-hsm
Signed-off-by: frank zago <fzago@cray.com>
Change-Id: If58546b9e23281097ed414f863e91b1e6a54c8fb
Reviewed-on: http://review.whamcloud.com/19578
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Vinayak <vinayakswami.hariharmath@seagate.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>