sg_init_table() should use unsigned loop index variable

Clean up: fix a mixed sign comparison in sg_init_table() accidentally
introduced by commit d6ec0842

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

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