Intel TSX-NI can cause a significant regression when TAA mitigation is needed

alessandro.vetere provided some performance testing results from a system where a transaction asynchronous abort (TAA) mitigation was enabled.

It would seem to be necessary to disable the use of hardware memory transactions on affected systems, at least in the function buf_page_optimistic_fix() but possibly across the board. A straightforward change on Linux would be to revise the detection function bool transactional_lock_enabled() so that it would avoid setting have_transactional_memory=true if the file /sys/devices/system/cpu/vulnerabilities/tsx_async_abort is readable and contains anything else than the string Not affected.