ANDROID: userfaultfd: Fix untag pointer in userfaultfd_continue()

Fixes e71e2ace ("userfaultfd: do not untag user pointers").

Above patch is ported from upstream into LTS branch then merged into
android12-5.4. The original patch was going to fix all untag user
pointers.

The change to userfaultfd_continue() was cut off when being merged into
LTS because the routine does not exist in LTS.

But specially the routine has been cherry-picked into android12-5.4 with
commit b69f713e ("BACKPORT: FROMGIT: userfaultfd: add
UFFDIO_CONTINUE ioctl") by Lokesh Gidra <lokeshgidra@google.com> long
ago.

So add back the missing part of fixing here.

Fixes: e71e2ace ("userfaultfd: do not untag user pointers")
Change-Id: I32da80c2e9517356daadf566a433e056f6bef08c
Signed-off-by: Orson Zhai <orson.zhai@unisoc.com>