nvme-fcloop: avoid possible uninitialized variable warning

nvme-fcloop: avoid possible uninitialized variable warning

The kbuild test robot send mail of a potential use of an uninitialized
variable - "tport" in fcloop_delete_targetport() which then calls
__targetport_unreg() which uses the variable. It will never be the
case it is uninitialized as the call to __targetport_unreg() only
occurs if there is a valid nport pointer. And at the time the nport
pointer is assigned, the tport variable is set.

Remove the warning by assigning a NULL value initially.

Signed-off-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>