nvme-multipath: fix leak on try_module_get failure

nvme-multipath: fix leak on try_module_get failure

We need to fall back to the synchronous removal if we can't get a
reference on the module needed for the deferred removal.

Fixes: 62188639ec16 ("nvme-multipath: introduce delayed removal of the multipath head node")
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>