mm/madvise: thread all madvise state through madv_behavior

Doing so means we can get rid of all the weird struct vm_area_struct
**prev stuff, everything becomes consistent and in future if we want to
make change to behaviour there's a single place where all relevant state
is stored.

This also allows us to update try_vma_read_lock() to be a little more
succinct and set up state for us, as well as cleaning up
madvise_update_vma().

We also update the debug assertion prior to madvise_update_vma() to assert
that this is a write operation as correctly pointed out by Barry in the
relevant thread.

We can't reasonably update the madvise functions that live outside of
mm/madvise.c so we leave those as-is.

Link: https://lkml.kernel.org/r/7b345ab82ef51e551f8bc0c4f7be25712871629d.1750433500.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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