Fix following possible bugs for SIOCSIFADDR, if in_ifinit() fails

Fix following possible bugs for SIOCSIFADDR, if in_ifinit() fails
Conditions:
   o  ifaceX has an AF_INET ia
   o  SIOCSIFADDR is used to change address, and new address' hash value is
      different from ia's
   o  ia is currently in hash bucket B1
   o  ia is removed from B1 and installed into hash table using new address
      hash value, assume its new hash bucket is B2, and B1 != B2
1) Dangling ia reference in inaddr hash table
   o  ifnet.if_ioctl fails
   o  ia is reinstalled into hash bucket B1, but without being first removed
      from hash bucket B2
   Hash bucket B2 will have a dangling reference to ia
2) ia is left in wrong hash bucket
   o  rtinit fails
   o  ia's address is restored to oldaddr
   ia itself is left in hash bucket indexed by new address's hash value

- In in_ifinit(), if it fails, unlink ia from inaddr hash table instead of
  delaying the unlinking to in_control_internal().  If necessary reinstall
  ia into inaddr hash table with original address
- After the above fix, in_control_internal() needs to unlink ia from inaddr
  only if cmd is SIOCDIFADDR and ia resides in inaddr hash table.  Whether
  ia is in inaddr hash table or not, is currently indicated by ia address's
  family; add XXX comment that this assumption is not good
- Constfy 'sin' parameter to in_ifinit()

Reviewed-by: dillon@