nvme: add vectored-io support for user-passthrough

Add a new NVME_IOCTL_IO64_CMD_VEC ioctl that works like the existing
NVME_IOCTL_IO64_CMD ioctl except that it takes and array of iovecs
and thus supports vectored I/O.

  - cmd.addr is base address of user iovec array
  - cmd.vec_cnt is count of iovec array elements

This patch does not include vectored-variant for admin-commands as most
of them are light on buffers and likely to have low invocation frequency.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>