pstore: fix pstore filesystem mount/remount issue

Currently after mount/remount operation on pstore filesystem,
the content on pstore will be lost. It is because current ERST
implementation doesn't support multi-user usage, which moves
internal pointer to the end after accessing it. Adding
multi-user support for pstore usage.

Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
diff --git a/include/linux/pstore.h b/include/linux/pstore.h
index 14ce2f5..2455ef2 100644
--- a/include/linux/pstore.h
+++ b/include/linux/pstore.h
@@ -35,6 +35,8 @@
 	struct mutex	buf_mutex;	/* serialize access to 'buf' */
 	char		*buf;
 	size_t		bufsize;
+	int		(*open)(struct pstore_info *psi);
+	int		(*close)(struct pstore_info *psi);
 	ssize_t		(*read)(u64 *id, enum pstore_type_id *type,
 			struct timespec *time);
 	u64		(*write)(enum pstore_type_id type, size_t size);