[NET]: sockfd_lookup_light() returns random error for -EBADFD

This applies to 2.6.17-rc2.

There is a missing initialization of err in sockfd_lookup_light() that
could return random error for an invalid file handle.

Signed-off-by: Hua Zhong <hzhong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>