svcrdma: avoid duplicate dma unmapping during error recovery

In rdma_read_chunk_frmr() when ib_post_send() fails, the error code path
invokes ib_dma_unmap_sg() to unmap the sg list. It then invokes
svc_rdma_put_frmr() which in turn tries to unmap the same sg list through
ib_dma_unmap_sg() again. This second unmap is invalid and could lead to
problems when the iova being unmapped is subsequently reused. Remove
the call to unmap in rdma_read_chunk_frmr() and let svc_rdma_put_frmr()
handle it.

Fixes: 412a15c0 ("svcrdma: Port to new memory registration API")
Cc: stable@vger.kernel.org
Signed-off-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>