gen_init_cpio: fixed fwrite warning

On compilers with security warnings enabled by default, we get:

usr/gen_init_cpio.c: In function ‘cpio_mkfile’:
usr/gen_init_cpio.c:357: warning: ignoring return value of ‘fwrite’,
                                  declared with attribute warn_unused_result

So check the return value and handle errors accordingly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>