x86/mm: always pass NULL as the first argument of switch_mm_irqs_off()

The first argument of switch_mm_irqs_off() is unused by the x86
implementation.  Make sure that x86 code never passes a non-NULL value to
make this clear.  Update the only non violating caller, switch_mm().

Link: https://lkml.kernel.org/r/20240222190911.1903054-2-yosryahmed@google.com


Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>