IB/core: Remove set-but-not-used variable from ib_sg_to_pages()

Detected this by building the IB core with W=1. See also patch
"IB core: Fix ib_sg_to_pages()" (commit 8f5ba10e).

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Christoph Hellwig <hch@lst.de>
Reviewed-by: Leon Romanovsky <leon.romanovsky@mellanox.com>
Acked-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>