NFSv4: Fix v4.0/v4.1 SEEK_DATA return -ENOTSUPP when set NFS_V4_2 config

Since commit bdcc2cd1 ("NFSv4.2: handle NFS-specific llseek errors"),
nfs42_proc_llseek would return -EOPNOTSUPP rather than -ENOTSUPP when
SEEK_DATA on NFSv4.0/v4.1.

This will lead xfstests generic/285 not run on NFSv4.0/v4.1 when set the
CONFIG_NFS_V4_2, rather than run failed.

Fixes: bdcc2cd1 ("NFSv4.2: handle NFS-specific llseek errors")
Cc: <stable.vger.kernel.org> # 4.2
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>