nvmet: centralize port enable access for configfs

The configfs attributes which are supposed to set when port is disable
such as addr[addrfam|portid|traddr|treq|trsvcid|inline_data_size|trtype]
has repetitive check and generic error message printing.

This patch creates centralize helper to check and print an error
message that also accepts caller as a parameter. This makes error
message easy to parse for the user, removes the duplicate code and
makes it available for futures such scenarios.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>