bloblist: Access record hdr_size and tag via a function

Convert accesses to tag and hdr_size via function for grouping tag and
hdr_size together later.

Signed-off-by: Simon Glass <sjg@chromium.org>
Co-developed-by: Raymond Mao <raymond.mao@linaro.org>
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>