pstore/ram: Refactor ftrace buffer merging

This changes the ftrace record merging code to be agnostic of
pstore/ram, as the first step to making it available as a generic
routine for other backends to use, such as pstore/zone.

Link: https://lore.kernel.org/lkml/20200510202436.63222-6-keescook@chromium.org/


Signed-off-by: Kees Cook <keescook@chromium.org>