mm: introduce vma_merge_struct and abstract vma_merge(),vma_modify()

Rather than passing around huge numbers of parameters to numerous helper
functions, abstract them into a single struct that we thread through the
operation, the vma_merge_struct ('vmg').

Adjust vma_merge() and vma_modify() to accept this parameter, as well as
predicate functions can_vma_merge_before(), can_vma_merge_after(), and the
vma_modify_...() helper functions.

Also introduce VMG_STATE() and VMG_VMA_STATE() helper macros to allow for
easy vmg declaration.

We additionally remove the requirement that vma_merge() is passed a VMA
object representing the candidate new VMA.  Previously it used this to
obtain the mm_struct, file and anon_vma properties of the proposed range
(a rather confusing state of affairs), which are now provided by the vmg
directly.

We also remove the pgoff calculation previously performed vma_modify(),
and instead calculate this in VMG_VMA_STATE() via the vma_pgoff_offset()
helper.

Link: https://lkml.kernel.org/r/a955aad09d81329f6fbeb636b2dd10cde7b73dab.1725040657.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: 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]>

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