MFV r287817: 6220 memleak in l2arc on debug build

https://github.com/illumos/illumos-gate/commit/c546f36aa898d913ff77674fb5ff97f15b2e08b4
https://www.illumos.org/issues/6220

5408 introduced a memleak in l2arc, namely the member b_thawed gets leaked when
an arc_hdr is realloced from full to l2only.

Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: George Wilson <george@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Arne Jansen <sensille@gmx.net>