Revert "net: micro-optimize skb_datagram_iter"

This reverts commit 934c29999b.
This triggered a usercopy BUG() in systems with HIGHMEM, reported
by the test robot in:
 https://lore.kernel.org/oe-lkp/202406161539.b5ff7b20-oliver.sang@intel.com

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://patch.msgid.link/20240626070153.759257-1-sagi@grimberg.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/datagram.c

@@ -417,14 +417,14 @@ static int __skb_datagram_iter(const struct sk_buff *skb, int offset,
 		end = start + skb_frag_size(frag);
 		if ((copy = end - offset) > 0) {
 			struct page *page = skb_frag_page(frag);
-			u8 *vaddr = kmap_local_page(page);
+			u8 *vaddr = kmap(page);
 
 			if (copy > len)
 				copy = len;
 			n = INDIRECT_CALL_1(cb, simple_copy_to_iter,
 					vaddr + skb_frag_off(frag) + offset - start,
 					copy, data, to);
-			kunmap_local(vaddr);
+			kunmap(page);
 			offset += n;
 			if (n != copy)
 				goto short_copy;