When a folio belongs exclusively to one process after a COW event,
folio_move_anon_rmap() always moves the folio into the anon_vma
belongs only to this process.
However, if the folio already belongs to the anon_vma of the this
process, we don't need to move it again. In this case, we first
check if the folio already belongs to the anna_vma of the this
process, and only move it if it does not.
The above changes may improve the performance of vm faults in some
scenarios, because the performance loss caused by WRITE_ONCE() is
much more than the performance loss caused by add a judgment.