selftests/mm: fix UFFDIO_API usage with proper two-step feature negotiation

selftests/mm: fix UFFDIO_API usage with proper two-step feature negotiation

The current implementation of test_unmerge_uffd_wp() explicitly sets
`uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP` before calling
UFFDIO_API.  This can cause the ioctl() call to fail with EINVAL on
kernels that do not support UFFD-WP, leading the test to fail
unnecessarily:

  # ------------------------------
  # running ./ksm_functional_tests
  # ------------------------------
  # TAP version 13
  # 1..9
  # # [RUN] test_unmerge
  # ok 1 Pages were unmerged
  # # [RUN] test_unmerge_zero_pages
  # ok 2 KSM zero pages were unmerged
  # # [RUN] test_unmerge_discarded
  # ok 3 Pages were unmerged
  # # [RUN] test_unmerge_uffd_wp
  # not ok 4 UFFDIO_API failed     <-----
  # # [RUN] test_prot_none
  # ok 5 Pages were unmerged
  # # [RUN] test_prctl
  # ok 6 Setting/clearing PR_SET_MEMORY_MERGE works
  # # [RUN] test_prctl_fork
  # # No pages got merged
  # # [RUN] test_prctl_fork_exec
  # ok 7 PR_SET_MEMORY_MERGE value is inherited
  # # [RUN] test_prctl_unmerge
  # ok 8 Pages were unmerged
  # Bail out! 1 out of 8 tests failed
  # # Planned tests != run tests (9 != 8)
  # # Totals: pass:7 fail:1 xfail:0 xpass:0 skip:0 error:0
  # [FAIL]

This patch improves compatibility and robustness of the UFFD-WP test
(test_unmerge_uffd_wp) by correctly implementing the UFFDIO_API two-step
handshake as recommended by the userfaultfd(2) man page.

Key changes:

1. Use features=0 in the initial UFFDIO_API call to query supported
   feature bits, rather than immediately requesting WP support.

2. Skip the test gracefully if:
   - UFFDIO_API fails with EINVAL (e.g. unsupported API version), or
   - UFFD_FEATURE_PAGEFAULT_FLAG_WP is not advertised by the kernel.

3. Close the initial userfaultfd and create a new one before enabling
   the required feature, since UFFDIO_API can only be called once per fd.

4. Improve diagnostics by distinguishing between expected and unexpected
   failures, using strerror() to report errors.

This ensures the test behaves correctly across a wider range of kernel
versions and configurations, while preserving the intended behavior on
kernels that support UFFD-WP.

[liwang@redhat.com: fail the test if sys_userfaultfd() fails, per David]
Link: https://lkml.kernel.org/r/20250625004645.400520-1-liwang@redhat.com
Link: https://lkml.kernel.org/r/20250624042411.395285-1-liwang@redhat.com
Signed-off-by: Li Wang <liwang@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Li Wang <liwang@redhat.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>