[$] splice() and the ghost of set_fs()

The normal rule of kernel development is that the creation of user-space
regressions is not allowed; a patch that breaks a previously working
application must be either fixed or reverted. There are exceptions,
though, including a
5.10 patch that has been turning up regressions ever since. The story
that emerges here shows what can happen when the goals of stability,
avoiding security problems, and code cleanup run into conflict.