OSD: expand_pg_num after pg removes

Otherwise:
1) expand_pg_num removes a splitting pg entry
2) peering thread grabs pg lock and starts split
3) OSD::consume_map grabs pg lock and starts removal

At step 2), we run afoul of the assert(is_splitting)
check in split_pgs.  This way, the would be splitting
pg is marked as removed prior to the splitting state
being updated.

Backport: bobtail
Fixes: #4449
Signed-off-by: Samuel Just <sam.just@inktank.com>