pstore doesn't support unregistering yet. It was marked as TODO.
This patch adds some code to fix it:
1) Add functions to unregister kmsg/console/ftrace/pmsg.
2) Add a function to free compression buffer.
3) Unmap the memory and free it.
4) Add a function to unregister pstore filesystem.

Signed-off-by: Geliang Tang
Acked-by: Kees Cook
[Removed __exit annotation from ramoops_remove(). Reported by Arnd Bergmann]
Signed-off-by: Tony Luck