initramfs: Refactor to use hex2bin() instead of custom approach

initramfs: Refactor to use hex2bin() instead of custom approach

There is a simple_strntoul() function used solely as a shortcut
for hex2bin() with proper endianess conversions. Replace that
and drop the unneeded function in the next changes.

This implementation will abort if we fail to parse the cpio header,
instead of using potentially bogus header values.

Co-developed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20260331070519.5974-5-ddiss@suse.de
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>