mm/rmap: remove anon_vma_merge() function

mm/rmap: remove anon_vma_merge() function

This function is confusing, we already have the concept of anon_vma merge
to adjacent VMA's anon_vma's to increase probability of anon_vma
compatibility and therefore VMA merge (see is_mergeable_anon_vma() etc.),
as well as anon_vma reuse, along side the usual VMA merge logic.

We can remove the anon_vma check as it is redundant - a merge would not
have been permitted with removal if the anon_vma's were not the same (and
in the case of an unfaulted/faulted merge, we would have already set the
unfaulted VMA's anon_vma to vp->remove->anon_vma in dup_anon_vma()).

Avoid overloading this term when we're very simply unlinking anon_vma
state from a removed VMA upon merge.

Link: https://lkml.kernel.org/r/56bbe45e309f7af197b1c4f94a9a0c8931ff2d29.1768746221.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chris Li <chriscli@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>