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 <ethan@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>