locking/lockdep: Fix TRACE_IRQFLAGS vs. NMIs

Prior to commit:

  859d069ee1 ("lockdep: Prepare for NMI IRQ state tracking")

IRQ state tracking was disabled in NMIs due to nmi_enter()
doing lockdep_off() -- with the obvious requirement that NMI entry
call nmi_enter() before trace_hardirqs_off().

[ AFAICT, PowerPC and SH violate this order on their NMI entry ]

However, that commit explicitly changed lockdep_hardirqs_*() to ignore
lockdep_off() and breaks every architecture that has irq-tracing in
it's NMI entry that hasn't been fixed up (x86 being the only fixed one
at this point).

The reason for this change is that by ignoring lockdep_off() we can:

  - get rid of 'current->lockdep_recursion' in lockdep_assert_irqs*()
    which was going to to give header-recursion issues with the
    seqlock rework.

  - allow these lockdep_assert_*() macros to function in NMI context.

Restore the previous state of things and allow an architecture to
opt-in to the NMI IRQ tracking support, however instead of relying on
lockdep_off(), rely on in_nmi(), both are part of nmi_enter() and so
over-all entry ordering doesn't need to change.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200727124852.GK119549@hirez.programming.kicks-ass.net

lib/Kconfig.debug

@@ -1325,11 +1325,17 @@ config WW_MUTEX_SELFTEST
 endmenu # lock debugging
 
 config TRACE_IRQFLAGS
+	depends on TRACE_IRQFLAGS_SUPPORT
 	bool
 	help
 	  Enables hooks to interrupt enabling and disabling for
 	  either tracing or lock debugging.
 
+config TRACE_IRQFLAGS_NMI
+	def_bool y
+	depends on TRACE_IRQFLAGS
+	depends on TRACE_IRQFLAGS_NMI_SUPPORT
+
 config STACKTRACE
 	bool "Stack backtrace support"
 	depends on STACKTRACE_SUPPORT