nfs: use vfs setgid helper

commit 4f704d9a upstream.

We've aligned setgid behavior over multiple kernel releases. The details
can be found in the following two merge messages:
cf619f89 ("Merge tag 'fs.ovl.setgid.v6.2')
426b4ca2 ("Merge tag 'fs.setgid.v6.0')
Consistent setgid stripping behavior is now encapsulated in the
setattr_should_drop_sgid() helper which is used by all filesystems that
strip setgid bits outside of vfs proper. Switch nfs to rely on this
helper as well. Without this patch the setgid stripping tests in
xfstests will fail.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230313-fs-nfs-setgid-v2-1-9a59f436cfc0@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
[ Harshit: backport to 5.15.y]
    fs/internal.h -- minor conflcit due to code change differences.
    include/linux/fs.h -- Used struct user_namespace *mnt_userns
                          instead of struct mnt_idmap *idmap
    fs/nfs/inode.c -- Used init_user_ns instead of nop_mnt_idmap ]
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>