mm/vmscan: Use vma iterator instead of vm_next

mm/vmscan: Use vma iterator instead of vm_next

Use the vma iterator in in get_next_vma() instead of the linked list.

Link: https://lkml.kernel.org/r/20220822150128.1562046-68-Liam.Howlett@oracle.com
Suggested-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>