io_uring_submit: fix head/tail wrap issue
io_uring_submit: fix head/tail wrap issue

Use proper unsigned math to figure out how many entries we have. If
we have head and tail on either side of UINT_MAX, then we currently
don't submit anything as:

while (head < tail) {

is never true.

Reported-by: Weiping Zhang <69f98136487e9d627cc48cf918f1523a7679b9f1@didiglobal.com>
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing