nfs: fix regression in handling of fsc= option in NFSv4

Setting the uniquifier for fscache via the fsc= mount
option is currently broken in NFSv4.

Fix this by passing fscache_uniq to root_fc if possible.

Signed-off-by: Chen Hanxiao <chenhx.fnst@fujitsu.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>