This patch adds checks to tcrypt to catch destination buffer
corruption for cipher tests across pages, which was useful
while fixing corruption caused by the HIFN driver.

It works by always clearing xbuf before the cipher operation
and counts how many bytes beyond the end of each buffer pointed
to by a scatterlist entry are non-zero.

The byte count is unreliable, if enough bytes are corrupted
it might reach the beginning of a new destination area and
include it in the count. I don't think this is important to
handle 100% correct since it usually shouldn't happen anyway,
but if this is considered a problem I would simply remove
the byte count entirely and only check the first byte.