LU-2703 osc: cancel lock outside of spinlock

during calling *_ap_completion, there is a ldlm lock needed to be
cancelled if the page belongs to readahead. this patch move the
cancellation out of the spin_lock cl_loi_list_lock for it is a big
operation containing lock acquisition (mutex) and memory allocation.

Change-Id: Iecd4c27d405ffa968cc74f264dfc48cb2e2f5671
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/5285
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>