IB/srp: Use pd->local_dma_lkey

Replace all leys with  pd->local_dma_lkey. This driver does not support
iWarp, so this is safe.

The insecure use of ib_get_dma_mr is thus isolated to an rkey, and will
have to be fixed separately.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>