pstore/ram: Factor dmesg przs initialization out of probe()

This will help make code clearer when we'll add support for other
message types.

This also makes probe() much shorter and understandable, plus
makes mem/record size checking a bit easier.

Implementation detail: we now use a paddr pointer, this will
be used for allocating persistent ram zones for other message
types.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>