crypto: serpent-sse2 - add xts support

crypto: serpent-sse2 - add xts support

Patch adds XTS support for serpent-sse2 by using xts_crypt(). Patch has been
tested with tcrypt and automated filesystem tests.

Tcrypt benchmarks results (serpent-sse2/serpent_generic speed ratios):

Intel Celeron T1600 (x86_64) (fam:6, model:15, step:13):
size    xts-enc xts-dec
16B     0.98x   1.00x
64B     1.00x   1.01x
256B    2.78x   2.75x
1024B   3.30x   3.26x
8192B   3.39x   3.30x

AMD Phenom II 1055T (x86_64) (fam:16, model:10):
size    xts-enc xts-dec
16B     1.05x   1.02x
64B     1.04x   1.03x
256B    2.10x   2.05x
1024B   2.34x   2.35x
8192B   2.34x   2.40x

Intel Atom N270 (i586):
size    xts-enc xts-dec
16B     0.95x   0.96x
64B     1.53x   1.50x
256B    1.72x   1.75x
1024B   1.88x   1.87x
8192B   1.86x   1.83x

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>