Illumos 5701 - zpool list reports incorrect "alloc" value for cache devices

5701 zpool list reports incorrect "alloc" value for cache devices
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Alek Pinchuk <alek.pinchuk@nexenta.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://www.illumos.org/issues/5701
https://github.com/illumos/illumos-gate/commit/a52fc31

Porting Notes:

arc_space_return(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);
correctly placed at arc_hdr_l2hdr_destroy(arc_buf_hdr_t *hdr).

Ported by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>