ofpbuf: Fix parameter for const initializer.

ofpbuf: Fix parameter for const initializer.

Clang 4.0 complains:

In file included from include/openvswitch/cxxtest.cc:11:0:
../include/openvswitch/ofpbuf.h: In function ‘ofpbuf ofpbuf_const_initializer(const void*, size_t)’:
../include/openvswitch/ofpbuf.h:107:5: warning: narrowing conversion of ‘size’ from ‘size_t {aka long unsigned int}’ to ‘uint32_t {aka unsigned int}’ inside { } [-Wnarrowing]
     };
     ^
../include/openvswitch/ofpbuf.h:107:5: warning: narrowing conversion of ‘size’ from ‘size_t {aka long unsigned int}’ to ‘uint32_t {aka unsigned int}’ inside { } [-Wnarrowing]

This is because the ofpbuf struct's "size" parameter is a uint32_t,
while ofpbuf_const_initializer() takes a size_t for the size. Fix this
function to take a uint32_t instead.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>