Before this diff, multiple calls to GCM_ENC_DEC will
succeed, but only if all calls are a multiple of 16 bytes.

Handle partial blocks at the start of GCM_ENC_DEC, and update
aadhash as appropriate.

The data offset %r11 is also updated after the partial block.
Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>