RDMA/umem: Don't check for a negative return value of dma_map_sg_attrs()

dma_map_sg_attrs() returns 0 on error and can't return a negative number
(ensured by BUG_ON), so don't check.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>