nvmet: pci-epf: cleanup link state management

Since the link_up boolean field of struct nvmet_pci_epf_ctrl is always
set to true when nvmet_pci_epf_start_ctrl() is called, assign true to
this field in nvmet_pci_epf_start_ctrl(). Conversely, since this field
is set to false when nvmet_pci_epf_stop_ctrl() is called, set this field
to false directly inside that function.

While at it, also add information messages to notify the user of the PCI
link state changes to help troubleshoot any link stability issues
without needing to enable debug messages.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>