NVMe: Add result to nvme_get_features

nvme_get_features() was not returning the result.  Add a parameter
to return the result in (similar to nvme_set_features()) and change
all callers.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>