The function blkcipher_get_spot tries to return a buffer of
the specified length that does not straddle a page.  It has
an off-by-one bug so it may advance a page unnecessarily.

What's worse, one of its callers doesn't provide a buffer
that's sufficiently long for this operation.

This patch fixes both problems.  Thanks to Bob Gilligan for
diagnosing this problem and providing a fix.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>