initramfs_test: add fill_cpio() inject_ox parameter

initramfs_test: add fill_cpio() inject_ox parameter

fill_cpio() uses sprintf() to write out the in-memory cpio archive from
an array of struct initramfs_test_cpio. This change allows callers to
modify the cpio sprintf() format string so that future tests can
intentionally corrupt the header with "0x" and "0X" prefixed fields.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://patch.msgid.link/20260331070519.5974-2-ddiss@suse.de
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>