cryptfs: Don't hardcode ascii buffer size

cryptfs: Don't hardcode ascii buffer size

We're removing hardcoded buffer sizes in anticipation of allowing
different keysizes.  In this case, our buffer was sufficiently
large for all current cases.  But if we ever changed the
crypt_mnt_ftr struct to allow larger keys, this code will adjust
with the change.

Bug: 73079191
Test: Flashed an encrypted sailfish and it booted.
Change-Id: I261e729a77b351e287fbb55327564fe512a23d47