mon/OSDMonitor: set CREATING flag for force-create-pg

mon/OSDMonitor: set CREATING flag for force-create-pg

In order to recreate a lost PG, we need to set the CREATING flag for the
pool.  This prevents pg_num from changing in future OSDMap epochs until
*after* the PG has successfully been instantiated.

Note that a pg_num change in *this* epoch is fine; the recreated PG will
instantiate in *this* epoch, which is /after/ the split a pg_num in this
epoch would describe.

Signed-off-by: Sage Weil <sage@redhat.com>