ovn-controller: Introduce helpers for looking up datapaths.

ovn-controller: Introduce helpers for looking up datapaths.

The new helpers get_local_datapath() and get_patched_datapath() make code
a little shorter and easier to read.  They also avoid a pitfall that was
present in at least a few of the instances: CONTAINER_OF is not safe on a
null pointer, because it does a raw pointer subtraction and will change
NULL to something else.  This wasn't actually a problem in these particular
cases because the value it was subtracting was zero (although arguably it
is still undefined behavior because the compiler is allowed to assume that
a pointer on which arithmetic is performed is nonnull).

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>