mm: userfaultfd: rename functions for clarity + consistency

Patch series "mm: userfaultfd: refactor and add UFFDIO_CONTINUE_MODE_WP",
v5.

- Commits 1-3 refactor userfaultfd ioctl code without behavior changes, with the
  main goal of improving consistency and reducing the number of function args.

- Commit 4 adds UFFDIO_CONTINUE_MODE_WP.


This patch (of 4):

The basic problem is, over time we've added new userfaultfd ioctls, and
we've refactored the code so functions which used to handle only one case
are now re-used to deal with several cases.  While this happened, we
didn't bother to rename the functions.

Similarly, as we added new functions, we cargo-culted pieces of the
now-inconsistent naming scheme, so those functions too ended up with names
that don't make a lot of sense.

A key point here is, "copy" in most userfaultfd code refers specifically
to UFFDIO_COPY, where we allocate a new page and copy its contents from
userspace.  There are many functions with "copy" in the name that don't
actually do this (at least in some cases).

So, rename things into a consistent scheme.  The high level idea is that
the call stack for userfaultfd ioctls becomes:

userfaultfd_ioctl
  -> userfaultfd_(particular ioctl)
    -> mfill_atomic_(particular kind of fill operation)
      -> mfill_atomic    /* loops over pages in range */
        -> mfill_atomic_pte    /* deals with single pages */
          -> mfill_atomic_pte_(particular kind of fill operation)
            -> mfill_atomic_install_pte

There are of course some special cases (shmem, hugetlb), but this is the
general structure which all function names now adhere to.

Link: https://lkml.kernel.org/r/20230314221250.682452-1-axelrasmussen@google.com
Link: https://lkml.kernel.org/r/20230314221250.682452-2-axelrasmussen@google.com


Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nadav Amit <namit@vmware.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>