sh: Flush executable pages in copy_user_highpage

This resolves a problem seen when using the Android dynamic linker.
Sometimes the dynamic linker would seg-fault at start up and this
was eventually traced to the handling of a COW fault for a page which
was being modified by the linker. If there was no cache aliasing between
the kernel and the user page, the page was not flushed, leaving the
newly copied data in the D-cache. However when executing instructions
from that page, the I-cache is filled directly from external memory,
rather than the D-cache, and causing garbage to be executed.

Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>