bloblist: check bloblist with specified buffer size

Instead of expecting the bloblist total size to be the same as the
pre-allocated buffer size, practically we are more interested in
whether the pre-allocated buffer size is bigger than the bloblist
total size.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>