crypto: sha256_ssse3 - fix stack corruption with SSSE3 and AVX implementations

The _XFER stack element size was set too small, 8 bytes, when it needs to be
16 bytes. As _XFER is the last stack element used by these implementations,
the 16 byte stores with 'movdqa' corrupt the stack where the value of register
%r12 is temporarily stored. As these implementations align the stack pointer
to 16 bytes, this corruption did not happen every time.

Patch corrects this issue.

Reported-by: Julian Wollrath <jwollrath@web.de>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Tested-by: Julian Wollrath <jwollrath@web.de>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>