openvswitch: Use kmalloc_size_roundup() to match ksize() usage

Round up allocations with kmalloc_size_roundup() so that openvswitch's
use of ksize() is always accurate and no special handling of the memory
is needed by KASAN, UBSAN_BOUNDS, nor FORTIFY_SOURCE.

Cc: Pravin B Shelar <pshelar@ovn.org>
Cc: dev@openvswitch.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221018090628.never.537-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>