netns: fix get_net_ns_by_fd(int pid) typo

The argument to get_net_ns_by_fd() is a /proc/$PID/ns/net file
descriptor not a pid.  Fix the typo.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Rami Rosen <roszenrami@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>