fs/pstore: update the backend parameter in pstore module

This patch update the module parameter backend, so it is visible
through /sys/module/pstore/parameters/backend.

For example:
if pstore backend is ramoops, with this patch:
	# cat /sys/module/pstore/parameters/backend
	ramoops
and without this patch:
	# cat /sys/module/pstore/parameters/backend
	(null)
Signed-off-by: NWang Long <long.wanglong@huawei.com>
Acked-by: NMark Salyzyn <salyzyn@android.com>
Signed-off-by: NTony Luck <tony.luck@intel.com>

fs/pstore/platform.c

@@ -456,6 +456,12 @@ int pstore_register(struct pstore_info *psi)
 		add_timer(&pstore_timer);
 	}
 
+	/*
+	 * Update the module parameter backend, so it is visible
+	 * through /sys/module/pstore/parameters/backend
+	 */
+	backend = psi->name;
+
 	pr_info("Registered %s as persistent store backend\n", psi->name);
 
 	return 0;