nfs: Properly initialize server->writeback

Atomic types should better be initialized with atomic_long_set() instead
of relying on zeroing done by kzalloc(). Clean this up.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>