Fix inaccurate arcstat_l2_hdr_size calculations

Based on the comments in arc.c we know that buffers can exist both in arc
and l2arc, under this circumstance both arc_buf_hdr_t and l2arc_buf_hdr_t
will be allocated. However the current logic only cares for memory that
l2arc_buf_hdr takes up when the buffer's state transfers from or to
arc_l2c_only. This will cause obvious deviations for illumos's zfs version
since the sizeof(l2arc_buf_hdr) is larger than ZOL's. We can implement
the calcuation in the following simple way:
1. When allocate a l2arc_buf_hdr_t we add its memory consumption instantly
and subtract it when we free or evict the l2arc buf.
2. According to the code in l2arc_hdr_stat_add and l2arc_hdr_stat_remove,
if the buffer only stays in l2arc we should also add the memory its arc_buf_hdr_t
consumes, so we only need to add HDR_SIZE to arcstat_l2_hdr_size since we already
concerned with L2HDR_SIZE in step 1 and vice vesa.

Signed-off-by: Ying Zhu <casualfisher@gmail.com>