Revert "gup: document and work around "COW can break either way" issue"

Revert "gup: document and work around "COW can break either way" issue"

This reverts commit 9bbd42e79720122334226afad9ddcac1c3e6d373, which
was commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.  The
backport was incorrect and incomplete:

* It forced the write flag on in the generic __get_user_pages_fast(),
  whereas only get_user_pages_fast() was supposed to do that.
* It only fixed the generic RCU-based implementation used by arm,
  arm64, and powerpc.  Before Linux 4.13, several other architectures
  had their own implementations: mips, s390, sparc, sh, and x86.

This will be followed by a (hopefully) correct backport.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>