pstore: Centralize init/exit routines

commit cb095afd44768bf495894b9ad063bd078e4bb201 upstream.

In preparation for having additional actions during init/exit, this moves
the init/exit into platform.c, centralizing the logic to make call outs
to the fs init/exit.

Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>