userfaultfd: selftest: exercise UFFDIO_COPY/ZEROPAGE -EEXIST

This will retry the UFFDIO_COPY/ZEROPAGE to verify it returns -EEXIST at
the first invocation and then later every 10 seconds.

In the filebacked MAP_SHARED case this also verifies the -EEXIST
triggered in the filesystem pagecache insertion, if the offset in the
file was not a hole.

shmem MAP_SHARED tries to index the newly allocated pagecache in the
radix tree before checking the pagetable so it doesn't need any
assistance to exercise that case.

hugetlbfs checks the pmd to be not none before trying to index the
hugetlbfs page in the radix tree, so it requires to run UFFDIO_COPY into
an alias mapping (the alternative would be to use MADV_DONTNEED to only
zap the pagetables, but that doesn't work on hugetlbfs).

[akpm@linux-foundation.org: fix uffdio_zeropage(), per Mike Kravetz]
Link: http://lkml.kernel.org/r/20170802165145.22628-3-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Alexey Perevalov <a.perevalov@samsung.com>
Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>