debugfs: write_file_bool() - ensure strtobool() operates on valid data

In case, userland writes an empty string to a bool debugfs file, buf[]
will still be uninitialized when being passed to strtobool() making the
outcome of that function purely random.

Fix this by always zero-terminating the buffer.

Signed-off-by: Mathias Krause <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

fs/debugfs/file.c

···
 	if (copy_from_user(buf, user_buf, buf_size))
 		return -EFAULT;
 
+	buf[buf_size] = '\0';
 	if (strtobool(buf, &bv) == 0)
 		*val = bv;