datatype: clamp boolean value to 0 and 1

datatype: clamp boolean value to 0 and 1

If user provides a numeric value larger than 0 or 1, match never
happens:

 # nft --debug=netlink add rule x y tcp option sack-perm 4
 ip x y
  [ exthdr load tcpopt 1b @ 4 + 0 present => reg 1 ]
  [ cmp eq reg 1 0x00000004 ]

After this update:

 # nft --debug=netlink add rule x y tcp option sack-perm 4
 ip x y
  [ exthdr load tcpopt 1b @ 4 + 0 present => reg 1 ]
  [ cmp eq reg 1 0x00000001 ]

This is to address a rare corner case, in case user specifies the
boolean value through the integer base type.

Fixes: 9fd9baba43c8 ("Introduce boolean datatype and boolean expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>