sh: __copy_user() optimizations for small copies.

This implements a fast-path for small (less than 12 bytes) copies,
with the existing path treated as the slow-path and left as the default
behaviour for all other copy sizes.

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