fs/proc/task_mmu: Fix loss of young/dirty bits during pagemap scan

make_uffd_wp_pte() was previously doing:

  pte = ptep_get(ptep);
  ptep_modify_prot_start(ptep);
  pte = pte_mkuffd_wp(pte);
  ptep_modify_prot_commit(ptep, pte);

But if another thread accessed or dirtied the pte between the first 2
calls, this could lead to loss of that information. Since
ptep_modify_prot_start() gets and clears atomically, the following is
the correct pattern and prevents any possible race. Any access after the
first call would see an invalid pte and cause a fault:

  pte = ptep_modify_prot_start(ptep);
  pte = pte_mkuffd_wp(pte);
  ptep_modify_prot_commit(ptep, pte);

Fixes: 52526ca7 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>