Revert "net: ptr_ring: otherwise safe empty checks can overrun array bounds"

This reverts commit bcecb4bb

.

If we try to allocate an extra entry as the above commit did, and when
the requested size is UINT_MAX, addition overflows causing zero size to
be passed to kmalloc().

kmalloc then returns ZERO_SIZE_PTR with a subsequent crash.

Reported-by:  <syzbot+87678bcf753b44c39b67@syzkaller.appspotmail.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>