nfs4: fix missing-braces warning

A bugfix introduced a harmless warning for update_open_stateid:

fs/nfs/nfs4proc.c:1548:2: error: missing braces around initializer [-Werror=missing-braces]

Removing the zero in the initializer will do the right thing here
and initialize the entire structure to zero.

Fixes: 1393d961 ("NFSv4: Fix a race when updating an open_stateid")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>