Let IORING_OP_FILES_UPDATE support to choose fixed file slots

Allocate available direct descriptors instead of having the
application pass free fixed file slots. To use it, pass
IORING_FILE_INDEX_ALLOC to io_uring_prep_files_update(), then
io_uring in kernel will store picked fixed file slots in fd
array and let cqe return the number of slots allocated.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220530173604.38000-1-xiaoguang.wang@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>