crypto: x86/sha1 - Fix reads beyond the number of blocks passed

It was reported that the sha1 AVX2 function(sha1_transform_avx2) is
reading ahead beyond its intended data, and causing a crash if the next
block is beyond page boundary:
http://marc.info/?l=linux-crypto-vger&m=149373371023377

This patch makes sure that there is no overflow for any buffer length.

It passes the tests written by Jan Stancek that revealed this problem:
https://github.com/jstancek/sha1-avx2-crash



I have re-enabled sha1-avx2 by reverting commit
b82ce244

Cc: <stable@vger.kernel.org>
Fixes: b82ce244 ("crypto: sha1-ssse3 - Disable avx2")
Originally-by: Ilya Albrekht <ilya.albrekht@intel.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Megha Dey <megha.dey@linux.intel.com>
Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>