Fix: ACCESS_ONCE() removed in kernel 4.15

Fix: ACCESS_ONCE() removed in kernel 4.15

The ACCESS_ONCE() macro was removed in kernel 4.15 and should be
replaced by READ_ONCE and WRITE_ONCE which were introduced in kernel
3.19.

This commit replaces all calls to ACCESS_ONCE() with the appropriate
READ_ONCE or WRITE_ONCE and adds compatibility macros for kernels that
have them.

See this upstream commit:

  commit b03a0fe0c5e4b46dcd400d27395b124499554a71
  Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
  Date:   Mon Oct 23 14:07:25 2017 -0700

    locking/atomics, mm: Convert ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

    For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
    preference to ACCESS_ONCE(), and new code is expected to use one of the
    former. So far, there's been no reason to change most existing uses of
    ACCESS_ONCE(), as these aren't currently harmful.

    However, for some features it is necessary to instrument reads and
    writes separately, which is not possible with ACCESS_ONCE(). This
    distinction is critical to correct operation.

    It's possible to transform the bulk of kernel code using the Coccinelle
    script below. However, this doesn't handle comments, leaving references
    to ACCESS_ONCE() instances which have been removed. As a preparatory
    step, this patch converts the mm code and comments to use
    {READ,WRITE}_ONCE() consistently.

    ----
    virtual patch

    @ depends on patch @
    expression E1, E2;
    @@

    - ACCESS_ONCE(E1) = E2
    + WRITE_ONCE(E1, E2)

    @ depends on patch @
    expression E;
    @@

    - ACCESS_ONCE(E)
    + READ_ONCE(E)
    ----

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>