initramfs: avoid memcpy for hex header fields

initramfs: avoid memcpy for hex header fields

newc/crc cpio headers contain a bunch of 8-character hexadecimal fields
which we convert via simple_strtoul(), following memcpy() into a
zero-terminated stack buffer. The new simple_strntoul() helper allows us
to pass in max_chars=8 to avoid zero-termination and memcpy().

Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20250304061020.9815-5-ddiss@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>