mm/mmap/vma_merge: initialize mid and next in natural order

It is more intuitive to go from prev to mid and then next.  No functional
change.

Link: https://lkml.kernel.org/r/20230309111258.24079-6-vbabka@suse.cz


Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>