ext4: fix NULL pointer dereference in print_daily_error_info()

Fix NULL pointer dereference in print_daily_error_info, when   
called on unmounted fs (EXT4_SB(sb) returns NULL), by removing error 
reporting timer in ext4_put_super.

Google-Bug-Id: 3017663
Signed-off-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -719,6 +719,7 @@ static void ext4_put_super(struct super_block *sb)
 			ext4_abort(sb, "Couldn't clean up the journal");
 	}
 
+	del_timer(&sbi->s_err_report);
 	ext4_release_system_zone(sb);
 	ext4_mb_release(sb);
 	ext4_ext_release(sb);