mm/mremap: complete refactor of move_vma()

We invoke ksm_madvise() with an intentionally dummy flags field, so no
need to pass around.

Additionally, the code tries to be 'clever' with account_start,
account_end, using these to both check that vma->vm_start != 0 and that we
ought to account the newly split portion of VMA post-move, either before
or after it.

We need to do this because we intentionally removed VM_ACCOUNT on the VMA
prior to unmapping, so we don't erroneously unaccount memory (we have
already calculated the correct amount to account and accounted it, any
subsequent subtraction will be incorrect).

This patch significantly expands the comment (from 2002!) about
'concealing' the flag to make it abundantly clear what's going on, as well
as adding and expanding a number of other comments also.

We can remove account_start, account_end by instead tracking when we
account (i.e.  vma->vm_flags has the VM_ACCOUNT flag set, and this is not
an MREMAP_DONTUNMAP operation), and figuring out when to reinstate the
VM_ACCOUNT flag on prior/subsequent VMAs separately.

We additionally break the function into logical pieces and attack the very
confusing error handling logic (where, for instance, new_addr is set to
err).

After this change the code is considerably more readable and easy to
manipulate.

Link: https://lkml.kernel.org/r/e7eaa307e444ba2b04d94fd985c907c8e896f893.1741639347.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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