Refactor some of the skb frag ref/unref helpers for improved clarity.

Implement napi_pp_get_page() to be the mirror counterpart of
napi_pp_put_page().

Implement skb_page_ref() to be the mirror of skb_page_unref().

Improve __skb_frag_ref() to become a mirror counterpart of
__skb_frag_unref(). Previously unref could handle pp & non-pp pages,
while the ref could only handle non-pp pages. Now both the ref & unref
helpers can correctly handle both pp & non-pp pages.

Now that __skb_frag_ref() can handle both pp & non-pp pages, remove
skb_pp_frag_ref(), and use __skb_frag_ref() instead.  This lets us
remove pp specific handling from skb_try_coalesce.

Additionally, since __skb_frag_ref() can now handle both pp & non-pp
pages, a latent issue in skb_shift() should now be fixed. Previously
this function would do a non-pp ref & pp unref on potential pp frags
(fragfrom). After this patch, skb_shift() should correctly do a pp
ref/unref on pp frags.
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20240410190505.1225848-3-almasrymina@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>