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

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