7867 ARC space accounting leak

illumos/illumos-gate@6de76ce2a90f54fecb0dba46dca08c99cef7aa08
https://github.com/illumos/illumos-gate/commit/6de76ce2a90f54fecb0dba46dca08c99cef7aa08

https://www.illumos.org/issues/7867

It seems that in the case where arc_hdr_free_pdata() sees HDR_L2_WRITING() we
would fail to update the ARC space statistics.
In the normal case those statistics are updated in arc_free_data_buf(). But in
the arc_hdr_free_on_write() path we don't do that.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Andriy Gapon <avg@FreeBSD.org>