In mpi_read_raw_from_sgl we may leak the SG miter resouces after
reading the leading zeroes.  This patch fixes this by stopping the
iteration once the leading zeroes have been read.

Fixes: 127827b9 ("lib/mpi: Do not do sg_virt")
Reported-by: NNicolai Stange <nicstange@gmail.com>
Tested-by: NNicolai Stange <nicstange@gmail.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>