net: allow skb_datagram_iter to be called from any context

We only use the mapping in a single context, so kmap_local is sufficient
and cheaper. Make sure to use skb_frag_foreach_page as skb frags may
contain compound pages and we need to map page by page.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202406161539.b5ff7b20-oliver.sang@intel.com
Fixes: 950fcaec

 ("datagram: consolidate datagram copy to iter helpers")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://patch.msgid.link/20240626100008.831849-1-sagi@grimberg.me
Signed-off-by: Paolo Abeni <pabeni@redhat.com>