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Ryan Roberts c36549ff8d Revert "x86/xen: allow nesting of same lazy mode"

Commit 49147beb0c ("x86/xen: allow nesting of same lazy mode") was added
as a solution for a core-mm code change where
arch_[enter|leave]_lazy_mmu_mode() started to be called in a nested
manner; see commit bcc6cc8325 ("mm: add default definition of
set_ptes()").

However, now that we have fixed the API to avoid nesting, we no longer
need this capability in the x86 implementation.

Additionally, from code review, I don't believe the fix was ever robust in
the case of preemption occurring while in the nested lazy mode.  The
implementation usually deals with preemption by calling
arch_leave_lazy_mmu_mode() from xen_start_context_switch() for the
outgoing task if we are in the lazy mmu mode.  Then in
xen_end_context_switch(), it restarts the lazy mode by calling
arch_enter_lazy_mmu_mode() for an incoming task that was in the lazy mode
when it was switched out.  But arch_leave_lazy_mmu_mode() will only unwind
a single level of nesting.  If we are in the double nest, then it's not
fully unwound and per-cpu variables are left in a bad state.

So the correct solution is to remove the possibility of nesting from the
higher level (which has now been done) and remove this x86-specific
solution.

Link: https://lkml.kernel.org/r/20250303141542.3371656-6-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Juergen Gross <jgross@suse.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juegren Gross <jgross@suse.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>