Shadowing is when a variable with a given name in an inner scope hides a
different variable with the same name in a surrounding scope.  This is
generally undesirable because it can confuse programmers.  This commit
eliminates most of it.

Found with -Wshadow=local in GCC 7.  The repo is not really ready to enable
this option by default because of a few cases that are harder to fix, and
harmless, such as nested use of CMAP_FOR_EACH.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>