x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK

A lot of ground work has been performed on x86 entry code. Fragile path
between user_enter() and user_exit() have IRQs disabled. Uses of RCU and
intrumentation in these fragile areas have been explicitly annotated
and protected.

This architecture doesn't need exception_enter()/exception_exit()
anymore and has therefore earned CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK.
Signed-off-by: NFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201117151637.259084-6-frederic@kernel.org

arch/x86/Kconfig

@@ -162,6 +162,7 @@ config X86
 	select HAVE_CMPXCHG_DOUBLE
 	select HAVE_CMPXCHG_LOCAL
 	select HAVE_CONTEXT_TRACKING		if X86_64
+	select HAVE_CONTEXT_TRACKING_OFFSTACK	if HAVE_CONTEXT_TRACKING
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS