Enable multiple XTS encryption or decryption

multiple xts_encrypt() cannot be performed because the
tweak is not updated. That means that
        xts_encrypt(buffer1, tweak)
        xts_encrypt(buffer2, tweak)
is not the same as
        xts_encrypt(concat(buffer1, buffer2), tweak)

Current patch enables such a functionality by
updating the tweak as output of the encryption.
Note that the tweak is no more constant.

The very same modification is performed
on xts_decrypt()

Signed-off-by: Pascal Brand <pascal.brand@st.com>