pstore: add pstore unregister

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 <geliangtang@163.com>
Acked-by: Kees Cook <keescook@chromium.org>
[Removed __exit annotation from ramoops_remove(). Reported by Arnd Bergmann]
Signed-off-by: Tony Luck <tony.luck@intel.com>
diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
index 916b8e2..360ae43 100644
--- a/fs/pstore/Kconfig
+++ b/fs/pstore/Kconfig
@@ -1,5 +1,5 @@
 config PSTORE
-	bool "Persistent store support"
+	tristate "Persistent store support"
 	default n
 	select ZLIB_DEFLATE
 	select ZLIB_INFLATE