pstore/ftrace: Provide ftrace log merging routine

Move the ftrace log merging logic out of pstore/ram into pstore/ftrace
so other backends can use it, like pstore/zone.

Link: https://lore.kernel.org/lkml/20200510202436.63222-7-keescook@chromium.org/


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