If rpcrdma_register_external() fails during request marshaling, the
current RPC request is killed. Instead, this RPC should be retried
after reconnecting the transport instance.

The most likely reason for registration failure with FRMR is a
failed post_send, which would be due to a remote transport
disconnect or memory exhaustion. These issues can be recovered
by a retry.

Problems encountered in the marshaling logic itself will not be
corrected by trying again, so these should still kill a request.

Now that we've added a clean exit for marshaling errors, take the
opportunity to defang some BUG_ON's.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>