xprtrdma: Use ro_unmap_sync in xprt_rdma_send_request

The "safe" version of ro_unmap is used here to avoid waiting
unnecessarily. However:

 - It is safe to wait. After all, we have to wait anyway when using
   FMR to register memory.

 - This case is rare: it occurs only after a reconnect.

By switching this call site to ro_unmap_sync, the final use of
ro_unmap_safe is removed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>