NFSv4.1: Clean ups for the device id cache

The fact that the global device id cache holds a reference to the
nfs4_deviceid_node until it is invisible to rcu lookups implies that
we can always assume that the reference count is non-zero in
_find_get_deviceid.

Also clean up nfs4_put_deviceid_node and the removal of the device id
from the cache.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>