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

The normal rule of kernel development is that the creation of user-spaceregressions is not allowed; a patch that breaks a previously workingapplication must be either fixed or reverted. There are exceptions,though, including a5.10 patch that has been turning up regressions ever since. The storythat emerges here shows what can happen when the goals of stability,avoiding security problems, and code cleanup run into conflict.