cmap: ovsrcu postpone the cmap destroy.

Currently, the cmap_destroy() directly frees the cmap memory.
Some callers of cmap_destroy() (e.g. destroy_subtable()) still
allows other threads (e.g. pmd threads) accessing the cmap at
the same time (e.g. via classifier_lookup_miniflow_batch()),
which could cause segfault.

To fix the above issue, this commit use ovsrcu to postpone
the free of cmap memory.

Reported-by: Ethan Jackson <[email protected]>
Signed-off-by: Alex Wang <[email protected]>
Acked-by: Ben Pfaff <[email protected]>

lib/cmap.c

@@ -240,7 +240,7 @@ void
 cmap_destroy(struct cmap *cmap)
 {
     if (cmap) {
-        free_cacheline(cmap_get_impl(cmap));
+        ovsrcu_postpone(free_cacheline, cmap_get_impl(cmap));
     }
 }