x86-splitlock-2021-06-28

 - Add the "ratelimit:N" parameter to the split_lock_detect= boot option,
   to rate-limit the generation of bus-lock exceptions. This is both
   easier on system resources and kinder to offending applications than
   the current policy of outright killing them.

 - Document the split-lock detection feature and its parameters.

Signed-off-by: Ingo Molnar <mingo@kernel.org>