bootconfig: Extend the magic check range to the preceding 3 bytes

Since Grub may align the size of initrd to 4 if user pass
initrd from cpio, we have to check the preceding 3 bytes as well.

Link: https://lkml.kernel.org/r/160520205132.303174.4876760192433315429.stgit@devnote2

Cc: stable@vger.kernel.org
Fixes: 85c46b78

 ("bootconfig: Add bootconfig magic word for indicating bootconfig explicitly")
Reported-by: Chen Yu <yu.chen.surf@gmail.com>
Tested-by: Chen Yu <yu.chen.surf@gmail.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>