xprtrdma: Don't defer MR recovery if ro_map fails

Deferred MR recovery does a DMA-unmapping of the MW. However, ro_map
invokes rpcrdma_defer_mr_recovery in some error cases where the MW
has not even been DMA-mapped yet.

Avoid a DMA-unmapping error replacing rpcrdma_defer_mr_recovery.

Also note that if ib_dma_map_sg is asked to map 0 nents, it will
return 0. So the extra "if (i == 0)" check is no longer needed.

Fixes: 42fe28f6 ("xprtrdma: Do not leak an MW during a DMA ...")
Fixes: 505bbe64

 ("xprtrdma: Refactor MR recovery work queues")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>