Fix arc_release() refcount

Update arc_release to use arc_buf_size(). This hunk was accidentally
dropped when porting compressed send/recv, 2aa34383b.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8000