nvme: only call synchronize_srcu when clearing current path

The function nmve_mpath_clear_current_path returns true if the current
path has changed. In this case we have to wait for all concurrent
submissions to finish. But if we didn't change the current path, there
is no point in waiting for another RCU period to finish.

Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>