LU-9579 osc: adds radix_tree_preload

The client fail with next error
osc_page_init()) ASSERTION( result == 0 )
in low memory conditions.

The patch fixes the problem above by adding
radix_tree_preload.

Signed-off-by: Alexander Boyko <alexander.boyko@seagate.com>
Seagate-bug-id: MRP-4424
Change-Id: I0a87366f8483fce4d2b3bcca449becce2a8262cb
Reviewed-on: https://review.whamcloud.com/27372
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>