page_pool: add a comment explaining the fragment counter usage

When reading the page_pool code the first impression is that keeping
two separate counters, one being the page refcnt and the other being
fragment pp_frag_count, is counter-intuitive.

However without that fragment counter we don't know when to reliably
destroy or sync the outstanding DMA mappings.  So let's add a comment
explaining this part.

Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/r/20230217222130.85205-1-ilias.apalodimas@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>