expression: expr_build_udata_recurse should recurse

expression: expr_build_udata_recurse should recurse

If we see EXPR_BINOP, recurse: ->left can be another EXPR_BINOP.

This is irrelevant for 'typeof' named sets, but for anonymous sets, the
key is derived from the concat expression that builds the lookup key for
the anonymous set.

 tcp option mptcp subtype . ip daddr { mp-join. 10.0.0.1, ..

 needs two binops back-to-back:

  [ exthdr load tcpopt 1b @ 30 + 2 => reg 1 ]
  [ bitwise reg 1 = ( reg 1 & 0x000000f0 ) ^ 0x00000000 ]
  [ bitwise reg 1 = ( reg 1 >> 0x00000004 ) ]

This bug prevents concat_expr_build_udata() from creating the userdata key
at load time.

When listing the rules, we get an assertion:
 nft: src/mergesort.c:23: concat_expr_msort_value: Assertion `ilen > 0' failed.

because the set has a key with 0-length integers.

Signed-off-by: Florian Westphal <fw@strlen.de>