NVMe scatterlists must be virtually contiguous, like almost all I/Os.
However, when the filesystem lays out files with a hole, it can be that
adjacent LBAs map to non-adjacent virtual addresses.  Handle this by
submitting one NVMe command at a time for each virtually discontiguous
range.
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>