nvme: export get and set features

Future use intends to make use of both, so export these functions. And
since their implementation is identical except for the opcode, provide a
new function that implement both.

[akinobu.mita@gmail.com>: fix line over 80 characters]
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>