sg_last() should use unsigned loop index variable

Clean up: fix a mixed sign comparison in sg_last() accidentally
introduced by commit 70eb8040

.  The sign of the loop index variable
should match the sign of the "nents" argument.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jens Axboe <axboe@carl.home.kernel.dk>