packet: refine ring v3 block size test to hold one frame

TPACKET_V3 stores variable length frames in fixed length blocks.
Blocks must be able to store a block header, optional private space
and at least one minimum sized frame.

Frames, even for a zero snaplen packet, store metadata headers and
optional reserved space.

In the block size bounds check, ensure that the frame of the
chosen configuration fits. This includes sockaddr_ll and optional
tp_reserve.

Syzbot was able to construct a ring with insuffient room for the
sockaddr_ll in the header of a zero-length frame, triggering an
out-of-bounds write in dev_parse_header.

Convert the comparison to less than, as zero is a valid snap len.
This matches the test for minimum tp_frame_size immediately below.

Fixes: f6fb8f10 ("af-packet: TPACKET_V3 flexible buffer implementation.")
Fixes: eb73190f ("net/packet: refine check for priv area size")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>