rgw: skip prefetch first chunk if range get falls to shadow objects

rgw: skip prefetch first chunk if range get falls to shadow objects

Currently the head object will be prefetched in each GET:
 a) This is unnecessary if the Range GET falls to shadow objects.
 b) The GET request would be quite slow if we have a big head object
This patch adds some check on the Range. If it's not in the head
object(>=rgw_max_chunk_size) then skip the prefetch.

Fixes: #12539
Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>