nvmet: fix per feat data len for get_feature

The existing implementation for the get_feature admin-cmd does not
use per-feature data len. This patch introduces a new helper function
nvmet_feat_data_len(), which is used to calculate per feature data len.
Right now we only set data len for fid 0x81 (NVME_FEAT_HOST_ID).

Fixes: commit e9061c39 ("nvmet: Remove the data_len field from the nvmet_req struct")

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Amit Engel <amit.engel@dell.com>
[endiness, naming, and kernel style fixes]
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>