Revert "FROMGIT: mm: improve mprotect(R|W) efficiency on pages referenced once"

This reverts commit b44e46bb.

Reason for revert:

The patch has not yet landed upstream, following feedback from Linus:
https://lore.kernel.org/all/CAHk-=wj4KCujAH_oPh40Bkp48amM4MXr+8AcbZ=qd5LF4Q+TDg@mail.gmail.com/#t



Bug: 213339151
Signed-off-by: Peter Collingbourne <pcc@google.com>
Change-Id: I81c2cef4076487df1dd0ee75449dcb2371ac1dbc
(cherry picked from commit ac448881)