mm/mremap: catch invalid multi VMA moves earlier

Previously, any attempt to solely move a VMA would require that the
span specified reside within the span of that single VMA, with no gaps
before or afterwards.

After commit d23cb648 ("mm/mremap: permit mremap() move of multiple
VMAs"), the multi VMA move permitted a gap to exist only after VMAs. 
This was done to provide maximum flexibility.

However, We have consequently permitted this behaviour for the move of
a single VMA including those not eligible for multi VMA move.

The change introduced here means that we no longer permit non-eligible
VMAs from being moved in this way.

This is consistent, as it means all eligible VMA moves are treated the
same, and all non-eligible moves are treated as they were before.

This change does not break previous behaviour, which equally would have
disallowed such a move (only in all cases).

[lorenzo.stoakes@oracle.com: do not incorrectly reference invalid VMA in VM_WARN_ON_ONCE()]
  Link: https://lkml.kernel.org/r/b6dbda20-667e-4053-abae-8ed4fa84bb6c@lucifer.local
Link: https://lkml.kernel.org/r/2b5aad5681573be85b5b8fac61399af6fb6b68b6.1754218667.git.lorenzo.stoakes@oracle.com


Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>