MFV r269542:

In vdev_get_stats, check that the vdev is not a hole before computing the
fragmentation.  This fixes a panic when removing log device.

Illumos issue:
    5049 panic when removing log device

Author:		Alex Reece <alex@delphix.com>
MFC after:	2 weeks

(cherry picked from commit a160f7bc63d16389899eca66d9a5847f9895493b)