[$] io_uring, SCM_RIGHTS, and reference-count cycles

The io_uring mechanism that was described here in
January has been through a number of revisions since then; those changes have
generally been fixing implementation issues rather than changing the
user-space API. In particular, this patch set seems to have received more
than the usual amount of security-related review, which can only be a good
thing. Security concerns became a bit of an obstacle for io_uring, though,
when virtual filesystem (VFS) maintainer Al Viro threatened
to veto the merging of the whole thing. It turns out that there were
some reference-counting issues that required his unique experience to
straighten out.