mm: simplify find_vma_prev()

mm: simplify find_vma_prev()

commit 297c5eee37 ("mm: make the vma list be doubly linked") added the
vm_prev member to vm_area_struct.  We can simplify find_vma_prev() by
using it.  Also, this change helps to improve page fault performance
because it has stronger locality of reference.

Signed-off-by: KOSAKI Motohiro <redacted>
Reviewed-by: KAMEZAWA Hiroyuki <redacted>
Cc: Hugh Dickins <redacted>
Cc: Peter Zijlstra <redacted>
Cc: Shaohua Li <redacted>
Cc: Michal Hocko <redacted>
Signed-off-by: Andrew Morton <redacted>
Signed-off-by: Linus Torvalds <redacted>