Revert "net: micro-optimize skb_datagram_iter"

This reverts commit 934c2999

.
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>