osd/PG: initialize info_struct_v in ctor

Otherwise we randomly assert on the first write_info() for newly created
PGs.  Introduced by 188f3ea6867eeb6e950f6efed18d53ff17522bbc.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge git://github.com/ceph/ceph