xprtrdma: Refactor rpcrdma_ia_open()

In order to unload a device driver and reload it, xprtrdma will need
to close a transport's interface adapter, and then call
rpcrdma_ia_open again, possibly finding a different interface
adapter.

Make rpcrdma_ia_open safe to call on the same transport multiple
times.

This is a refactoring change only.

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