rpcrdma: Always release the rpcrdma_device's xa_array

Dai pointed out that the xa_init_flags() in rpcrdma_add_one() needs
to have a matching xa_destroy() in rpcrdma_remove_one() to release
underlying memory that the xarray might have accrued during
operation.

Reported-by: Dai Ngo <dai.ngo@oracle.com>
Fixes: 7e86845a ("rpcrdma: Implement generic device removal")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>