RDMA/rw: Fix error flow during RDMA context initialization

RDMA/rw: Fix error flow during RDMA context initialization

In case the SGL was mapped for P2P DMA operation, we must unmap it using
pci_p2pdma_unmap_sg during the error unwind of rdma_rw_ctx_init()

Fixes: 43fc5da7606e ("PCI/P2PDMA: Introduce pci_p2pdma_unmap_sg()")
Link: https://lore.kernel.org/r/20200220100819.41860-1-maxg@mellanox.com
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>