lib/zlib: fix DFLTCC ignoring flush modes when avail_in == 0

This commit is based on:
  https://github.com/zlib-ng/zlib-ng/commit/40acb3f

Link: https://lkml.kernel.org/r/20230126131428.1222214-5-zaslonko@linux.ibm.com


Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>