Illumos 4897

Illumos 4897 - Space accounting mismatch in L2ARC/zpool
    
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>
    
From the illumos issue tracker:
    
    L2ARC vdev space usage statistics are calculated as the delta
    between the maximum and minimum vdev offset ever written to
    by the L2ARC fill thread, but do not inform the user of how 
    much space in between these two offsets is actually taken up by
    cached buffers. This fix changes that so that vdev space usage
    stats on L2ARC devices accurately track the volume of buffers
    stored on them, allowing users to see the exact L2ARC usage in
    "zpool iostat -v".
    
References:
    https://www.illumos.org/issues/4897
Ported by: Tim Chase <tim@chase2k.com>