ramoops: use pstore interface

Instead of using /dev/mem directly and forcing userspace to know (or
extract) where the platform has defined persistent memory, how many slots
it has, the sizes, etc, use the common pstore infrastructure to handle
Oops gathering and extraction.  This presents a much easier to use
filesystem-based view to the memory region.  This also means that any
other tools that are written to understand pstore will automatically be
able to process ramoops too.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Marco Stornelli <marco.stornelli@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>