OSD: cancel_pending_splits needs to cancel all descendants

expand_pg_num() and load_pgs() may result in a pg with children
in pending_splits which also have children in pending_splits (etc).

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: load_pgs() should fill in start_split honestly

In load_pgs(), we previously called assigned children starting
at the loaded pg created between its stored epoch and the current
osdmap to have that pg as their parent.  This is not correct, some
of the children may have been split in subsequent epochs from children
split in earlier epochs.  Instead, do each map individually.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branch 'upstream/wip_4884' into next

Fixes: #4884
Reviewed-by: Greg Farnum <greg@inktank.com>