osd/PGLog: reindex properly on pg log split

osd/PGLog: reindex properly on pg log split

When pg_log_t::split_out_child() runs it builds the list, which means the
old indexes are wrong (the point to bad memory), but index() will not
rebuild them because ever since b858e869e78927dccebaa350d246bd74af7f1de9
we won't rebuild them if they are already built.

Fix that by calling unindex() before the split.

Further, the new child log also needs to be indexed.  Fix that too.

Fixes: http://tracker.ceph.com/issues/18975
Signed-off-by: Sage Weil <sage@redhat.com>