crypto: chelsio - This fixes the kernel panic which occurs during a libkcapi test

The libkcapi test which causes kernel panic is
aead asynchronous vmsplice multiple test.

./bin/kcapi  -v -d 4 -x 10   -c "ccm(aes)"
-q 4edb58e8d5eb6bc711c43a6f3693daebde2e5524f1b55297abb29f003236e43d
-t a7877c99 -n 674742abd0f5ba -k 2861fd0253705d7875c95ba8a53171b4
-a fb7bc304a3909e66e2e0c5ef952712dd884ce3e7324171369f2c5db1adc48c7d

This patch avoids dma_mapping of a zero length sg which causes the panic,
by using sg_nents_for_len which maps only upto a specific length

Signed-off-by: Ayush Sawal <ayush.sawal@chelsio.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>