Revert "io_uring/uring_cmd: defer SQE copying until it's needed"

JIRA: https://issues.redhat.com/browse/RHEL-82698


CVE: CVE-2025-21837
Upstream status: RHEL-only

This reverts commit 5eff57fa.

The upstream fix was different because of follow-on fixes to the buggy
commit.  Rather than backporting more incomplete fixes, just revert the
source of the problem.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>