NFSD: Initialize pointer ni with NULL and not plain integer 0

[ Upstream commit 8e70bf27 ]

Pointer ni is being initialized with plain integer zero. Fix
this by initializing with NULL.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>