skbuff: constify skb_propagate_pfmemalloc() "page" argument

The function doesn't write anything to the page struct itself,
so this argument can be const.

Misc: align second argument to the brace while at it.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>