nvme_fc: add 'nvme_discovery' sysfs attribute to fc transport device

The fc transport device should allow for a rediscovery, as userspace
might have lost the events. Example is udev events not handled during
system startup.

This patch add a sysfs entry 'nvme_discovery' on the fc class to
have it replay all udev discovery events for all local port/remote
port address pairs.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>