mm: userfaultfd: add UFFDIO_CONTINUE_MODE_WP to install WP PTEs

UFFDIO_COPY already has UFFDIO_COPY_MODE_WP, so when installing a new PTE
to resolve a missing fault, one can install a write-protected one.  This
is useful when using UFFDIO_REGISTER_MODE_{MISSING,WP} in combination.

This was motivated by testing HugeTLB HGM [1], and in particular its
interaction with userfaultfd features.  Existing userfaultfd code supports
using WP and MINOR modes together (i.e.  you can register an area with
both enabled), but without this CONTINUE flag the combination is in
practice unusable.

So, add an analogous UFFDIO_CONTINUE_MODE_WP, which does the same thing as
UFFDIO_COPY_MODE_WP, but for *minor* faults.

Update the selftest to do some very basic exercising of the new flag.

Update Documentation/ to describe how these flags are used (neither the
COPY nor the new CONTINUE versions of this mode flag were described there
before).

[1]: https://patchwork.kernel.org/project/linux-mm/cover/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Axel Rasmussen <[email protected]>
Acked-by: Peter Xu <[email protected]>
Acked-by: Mike Rapoport (IBM) <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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