alpha: fix the local TLB invalidate in flush_tlb_page()

flush_tlb_page() invalidates the calling CPU itself before asking the
others, and gates that on current->active_mm. For a non-executable vma
that means a targeted tbi(2, addr), which acts on the context currently
loaded and is only guaranteed to invalidate the intended translations
when the target mm's context is the loaded one. A lazy active_mm does
not establish that, so as in ipi_flush_tlb_page() the target mm's stale
translations can survive, and nothing forces the old ASN to be retired
afterwards.

Test current->mm instead. A lazy caller then takes the flush_tlb_other()
branch added by the previous patch, retiring its local context. Adding
that branch first preserves the local invalidate at every step, including
on EV7 where a targeted tbi() against a foreign ASN appeared to work.

Which callers reach here with a foreign mm depends on the configuration.
folio_mkclean(), from the writeback flusher kworker that has no mm of its
own, accounted for about half the calls during writeback of a shared
mapping and none at all on anonymous memory. Those counts were measured
with CONFIG_COMPACTION=n: with COMPACTION=y, asm/pgtable.h overrides
ptep_clear_flush() to call migrate_flush_tlb_page(), which rendezvouses
with every CPU and handles the context itself, so folio_mkclean() does
not reach this function.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20261010142533.974797-6-linmag7@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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