I got a problem on MIPS with Big-Endian is turned on: every time when
NF trying to change TCP MSS it returns because of new.v16 was greater
than old.v16. But real MSS was 1460 and my rule was like this:

	add rule table chain tcp option maxseg size set 1400

And 1400 is lesser that 1460, not greater.

Later I founded that main causer is cast from u32 to __be16.

Debugging:

In example MSS = 1400(HEX: 0x578). Here is representation of each byte
like it is in memory by addresses from left to right(e.g. [0x0 0x1 0x2
0x3]). LE — Little-Endian system, BE — Big-Endian, left column is type.

	     LE               BE
	u32: [78 05 00 00]    [00 00 05 78]

As you can see, u32 representation will be casted to u16 from different
half of 4-byte address range. But actually nf_tables uses registers and
store data of various size. Actually TCP MSS stored in 2 bytes. But
registers are still u32 in definition:

	struct nft_regs {
		union {
			u32			data[20];
			struct nft_verdict	verdict;
		};
	};

So, access like regs->data[priv->sreg] exactly u32. So, according to
table presents above, per-byte representation of stored TCP MSS in
register will be:

	                     LE               BE
	(u32)regs->data[]:   [78 05 00 00]    [05 78 00 00]
	                                       ^^ ^^

We see that register uses just half of u32 and other 2 bytes may be
used for some another data. But in nft_exthdr_tcp_set_eval() it casted
just like u32 -> __be16:

	new.v16 = src

But u32 overfill __be16, so it get 2 low bytes. For clarity draw
one more table(<xx xx> means that bytes will be used for cast).

	                     LE                 BE
	u32:                 [<78 05> 00 00]    [00 00 <05 78>]
	(u32)regs->data[]:   [<78 05> 00 00]    [05 78 <00 00>]

As you can see, for Little-Endian nothing changes, but for Big-endian we
take the wrong half. In my case there is some other data instead of
zeros, so new MSS was wrongly greater.

For shooting this bug I used solution for ports ranges. Applying of this
patch does not affect Little-Endian systems.

Signed-off-by: Sergey Marinkevich <sergey.marinkevich@eltex-co.ru>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>