mm: refactor vma_merge() into modify-only vma_merge_existing_range()

The existing vma_merge() function is no longer required to handle what
were previously referred to as cases 1-3 (i.e.  the merging of a new VMA),
as this is now handled by vma_merge_new_vma().

Additionally, simplify the convoluted control flow of the original,
maintaining identical logic only expressed more clearly and doing away
with a complicated set of cases, rather logically examining each possible
outcome - merging of both the previous and subsequent VMA, merging of the
previous VMA and merging of the subsequent VMA alone.

We now utilise the previously implemented commit_merge() function to share
logic with vma_expand() de-duplicating code and providing less surface
area for bugs and confusion.  In order to do so, we adjust this function
to accept parameters specific to merging existing ranges.

Link: https://lkml.kernel.org/r/2cf6016b7bfcc4965fc3cde10827560c42e4f12c.1725040657.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Bert Karwatzki <[email protected]>
Cc: Jeff Xu <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Paul Moore <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>