sh: Handle calling csum_partial with misaligned data

In rare circumstances csum_partial() can be called with data which is
not 16 or 32 bit aligned. This is been observed with RPC calls for NFS
file systems for example. Add support for handling this without resorting
to the misaligned fixup code (which is why this hasn't been seen as a
problem). This mimics the i386 version, which has had this support for
some time.

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