tpm: Rename tpm2_allow_extend()

tpm: Rename tpm2_allow_extend()

When that function was introduced we were only using it to check if
extending a PCR was allowed, so the name made sense. A few patches ago
we used that function to reason about the EventLog creation and general
usage of PCRs , so let's rename it to something more generic that makes
more sense in all contexts.

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