hammer - Really fix VFS_STATFS() when operating volumes.

- Correct a mistake in the previous "fix". Compute the
  total number of blocks based on the bigblock count which
  was already updated.
- Take care of stats also on volume deletion.

Dragonfly-bug: <http://bugs.dragonflybsd.org/issue2497>
kernel/nataraid: Add devstat support.
kernel/nataraid: Fix a bug for array sizes >2TB.

The overall array size (total_sectors) in the softc was already 64 bit
wide but due to a missing cast when multiplying the 32 bit disk size
by the number of disks, it never became larger than 32 bits.

Also, the disk size was signed when it should have been unsigned.

Note that these fixes apply to RAIDs created using natacontrol(8), but
not necessarily to those created with BIOS utilities.

Reported-by: Aaron Bieber <deftly@gmail.com>