flow: Enforce that freeing of closed flows must happen in deferred handlers

Currently, flows are only evern finally freed (and the table compacted)
from the deferred handlers.  Some future ways we want to optimise managing
the flow table will rely on this, so enforce it: rather than having the
TCP code directly call flow_table_compact(), add a boolean return value to
the per-flow deferred handlers.  If true, this indicates that the flow
code itself should free the flow.

This forces all freeing of flows to occur during the flow code's scan of
the table in flow_defer_handler() which opens possibilities for future
optimisations.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

flow.h

@@ -68,7 +68,6 @@ static inline bool flow_sidx_eq(flow_sidx_t a, flow_sidx_t b)
 
 union flow;
 
-void flow_table_compact(const struct ctx *c, union flow *hole);
 void flow_defer_handler(const struct ctx *c, const struct timespec *now);
 
 void flow_log_(const struct flow_common *f, int pri, const char *fmt, ...)