classifier: Fix race condition leading to NULL dereference.

classifier: Fix race condition leading to NULL dereference.

Addition of table versioning exposed struct cls_rule member
'cls_match' to RCU readers and made it possible for 'cls_match' become
NULL while being accessed by an RCU reader, but we failed to check for
this condition.  This may have resulted in NULL pointer dereference
and ovs-vswitchd crash.

Fix this by making the 'cls_match' member an RCU pointer and checking
the value whenever it potentially read by an RCU reader.  In these
instances we use ovsrcu_get(), whereas functions accessible only by
the exclusive writers use ovsrcu_get_protected() and do not need to
check the result.

VMware-BZ: 1643642
Fixes: 2b7b1427 ("classifier: Support table versioning")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>