skbuff: Replace open-coded skb_propagate_pfmemalloc()s

Use skb_propagate_pfmemalloc() in build_skb()/build_skb_around() instead
of open-coding it.

Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>