x86/mm: clarify "prev" usage in switch_mm_irqs_off()

In the x86 implementation of switch_mm_irqs_off(), we do not use the
"prev" argument passed in by the caller, we use exclusively use
"real_prev", which is cpu_tlbstate.loaded_mm.  This is not obvious at the
first sight.

Furthermore, a comment describes a condition that happens when called with
prev == next, but this should not affect the function in any way since
prev is unused.  Apparently, the comment is intended to clarify why we
don't rely on prev == next to decide whether we need to update CR3, but
again, it is not obvious.  The comment also references the fact that
leave_mm() calls with prev == NULL and tsk == NULL, but this also
shouldn't matter because prev is unused and tsk is only used in one
function which has a NULL check.

Clarify things by renaming (prev -> unused) and (real_prev -> prev), also
move and rewrite the comment as an explanation for why we don't rely on
"prev" supplied by the caller in x86 code and use our own.  Hopefully this
makes reading the code easier.

Link: https://lkml.kernel.org/r/20240126080644.1714297-2-yosryahmed@google.com


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