gen_init_cpio: write to fd instead of stdout stream

In preparation for more efficient archiving using copy_file_range(),
switch from writing archive data to stdout to using STDOUT_FILENO and
I/O via write(), dprintf(), etc.
Basic I/O error handling is added to cover cases such as ENOSPC. Partial
writes are treated as errors.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250819032607.28727-2-ddiss@suse.de


Signed-off-by: Nathan Chancellor <nathan@kernel.org>