nfsd: use DEFINE_SHOW_ATTRIBUTE to define export_features_fops and supported_enctypes_fops

[ Upstream commit 9beeaab8 ]

Use DEFINE_SHOW_ATTRIBUTE helper macro to simplify the code.

Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
[ cel: reduce line length ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>