NVMe: Remove duplicate compat SG_IO code

We can return -ENOIOCTLCMD and the ioctl will be handled by
fs/compat_ioctl.c instead. This removes a lot of duplicate code in the
nvme driver.

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