gen_init_cpio: support -o <output_file> parameter

This is another preparatory change to allow for reflink-optimized
cpio archives with file data written / cloned via copy_file_range().
The output file is truncated prior to write, so that it maps to
usr/gen_initramfs.sh usage. It may make sense to offer an append option
in future, for easier archive concatenation.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250819032607.28727-3-ddiss@suse.de


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