ANDROID: mm: fix vma refcounting for SPF in vm_area_dup

vm_area_dup() should reset the vma refcount to avoid leaking the newly
created vma in case the original vma being copies has an elevated
refcount due to concurrent page fault.

Fixes: d9d7f843 ("ANDROID: mm: introduce vma refcounting to protect vma during SPF")
Bug: 253557903
Change-Id: I11324ac5661823e9c97b71afe6a894a094439960
Signed-off-by: Suren Baghdasaryan <surenb@google.com>