This patch adds support for skipping a file descriptor when using
IORING_REGISTER_FILES_UPDATE.  __io_sqe_files_update will skip fds set
to IORING_REGISTER_FILES_SKIP. IORING_REGISTER_FILES_SKIP is inturn
added as a #define in io_uring.h
Signed-off-by: Nnoah <goldstein.w.n@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>