OSD::_try_resurrect_pg: fix cur/pgid confusion

This bug prevented resurrection of ancestor pgs where
necessary.

Fixes: #5269
This may result in pg A being created just before pg B
is resurrected and split into A and B resulting in one
or the other operations getting and EEXIST.

Backport: cuttlefish
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>