Fix: info_struct_v updated in PG::_write_info
Description: Avoid unneccessary pg_biginfo updating, info_struct_v should pass by reference. Otherwise, when PG is first created, info_struct_v is always zero and cannot be updated to the right version. Dirty_big_info is always true and does unneccessary pg_biginfo updating before next time we restart osd daemon.

Signed-off-by: Ning Yao <zay11022@gmail.com>