Revert "net: Remove low_thresh in ip defrag"

This reverts commit b2cbac9b

.

We have multiple reports of obvious breakage from this patch.

Reported-by: Ido Schimmel <idosch@idosch.org>
Link: https://lore.kernel.org/all/ZGIRWjNcfqI8yY8W@shredder/
Link: https://lore.kernel.org/all/CADJHv_sDK=0RrMA2FTZQV5fw7UQ+qY=HG21Wu5qb0V9vvx5w6A@mail.gmail.com/
Reported-by:  <syzbot+a5e719ac7c268e414c95@syzkaller.appspotmail.com>
Reported-by:  <syzbot+a03fd670838d927d9cd8@syzkaller.appspotmail.com>
Fixes: b2cbac9b

 ("net: Remove low_thresh in ip defrag")
Link: https://lore.kernel.org/r/20230517034112.1261835-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>