Description of problem:
On Linux values ipAdEntIfIndex values for other than 127.0.0.1 point
to the wrong IfIndex. 

The Attached patch fixes the problem mentioned above. It works okay on
both 2.4 and 2.6 kernels along with 1-4 physical interfaces besides lo0.

Since ipAddr.c pretty much blindly takes the ifreq->ifr_index value
coming filled by the kernel I guess the real problem might not be with
net-snmp. Once upon the someone had to write their own interface index
lookup routines in net-snmp for linux. My patch ignores both of these
(Address_Scan_Init() and Address_Scan_next()) and instead uses the
functions available for other operating systems, mostly *BSD I guess.
There is also a check to ignore addresses from 0/8 as interfaces
without an address get assigned 0.0.0.0.

It took a while to reduce it to only a few lines of changes ;-)

Version-Release number of selected component (if applicable):
net-snmp-5.1.1

How reproducible:
Always

Steps to Reproduce:
1. snmpwalk -v 2c -c community host ifdescr
2. snmpwalk -v 2c -c community host ipadentifindex
3. 
  
Actual results:
ifDescr.1 = STRING: "lo"
ifDescr.2 = STRING: "eth0"
ifDescr.3 = STRING: "eth1"
ifDescr.4 = STRING: "sit0"
ipAdEntIfIndex.127.0.0.1 = INTEGER: 1
ipAdEntIfIndex.193.65.56.49 = INTEGER: 4
ipAdEntIfIndex.193.94.142.90 = INTEGER: 5

Expected results:
ifDescr.1 = STRING: "lo"
ifDescr.2 = STRING: "eth0"
ifDescr.3 = STRING: "eth1"
ifDescr.4 = STRING: "sit0"
ipAdEntIfIndex.127.0.0.1 = INTEGER: 1
ipAdEntIfIndex.193.65.56.49 = INTEGER: 2
ipAdEntIfIndex.193.94.142.90 = INTEGER: 3


Additional info:
RFC1213 states: "The index value which uniquely identifies the
interface to which this entry is applicable.  The interface identified
by a particular value of this index is the same interface as
identified by the same value of ifIndex."

Thanks.