Fix arc_size accounting mistake

l2arc_hdr_stat_[add|remove] need to be updated to reflect the fact
that L2ARC headers are no longer being accounted for in arc_size
nor arc_meta_used.

This should be folded into the original patch which removed the l2arc
headers from those accounting variables in the first place (ffaebeb).
That original patch is essentially broken without this fix.

Signed-off-by: Prakash Surya <surya1@llnl.gov>