nfs: avoid dereferencing null pointer in initiate_bulk_draining

Fix an inverted null pointer check in initiate_bulk_draining().

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org [>= 3.7]