initramfs: Check negative timestamp to prevent broken cpio archive

Similar to commit 4c9d410f

 ("initramfs: Check timestamp to prevent
broken cpio archive"), except asserts that the timestamp is
non-negative. This can happen when the KBUILD_BUILD_TIMESTAMP is a value
before UNIX epoch, which may be set when making reproducible builds that
don't want to look like they use a valid date.

While support for dates before 1970 might not be supported, this is more
about preventing undetected CPIO corruption. The printf's use a minimum
length format specifier, and will happily make the field longer than 8
characters if they need to.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Tested-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>