[$] SA_IMMUTABLE and the hazards of messing with signals

There are some parts of the kernel where even the most experienced andcapable developers fear to tread; one of those is surely the code thatimplements signals. The nature of the signal API almost guarantees thatany implementation will be full of subtle interactions and complexities,and the version in Linux doesn't disappoint. So the inclusion of asignal-handling change late in the 5.16 merge window might have beenexpected to have the potential for difficulties; it didn't disappointeither.