pstore/ram: Refactor DT size parsing

pstore/ram: Refactor DT size parsing

Refactor device tree size parsing routines to be able to pass a non-zero
default value for providing a configurable default for the coming
"max_reason" field. Also rename the helpers, since we're not always
parsing a size -- we're parsing a u32 and making sure it's not greater
than INT_MAX.

Link: https://lore.kernel.org/lkml/20200506211523.15077-4-keescook@chromium.org/
Link: https://lore.kernel.org/lkml/20200521205223.175957-1-tyhicks@linux.microsoft.com
Signed-off-by: Kees Cook <keescook@chromium.org>