idmapped-mounts: Add mknodat operation in setgid test

idmapped-mounts: Add mknodat operation in setgid test

Since mknodat can create file, we should also check whether strip S_ISGID.
Also add new helper caps_down_fsetid to drop CAP_FSETID because strip S_ISGID
depend on this cap and keep other cap(ie CAP_MKNOD) because create character
device needs it when using mknod.

Only test mknodat with character device in setgid_create function and the another
two functions test mknodat with whiteout device.

Since kernel commit a3c751a50 ("vfs: allow unprivileged whiteout creation") in
v5.8-rc1, we can create whiteout device in userns test. Since kernel 5.12, mount_setattr
and MOUNT_ATTR_IDMAP was supported, we don't need to detect kernel whether allow
unprivileged whiteout creation. Using fs_allow_idmap as a proxy is safe.

Tested-by: Christian Brauner (Microsoft)<brauner@kernel.org>
Reviewed-by: Christian Brauner (Microsoft)<brauner@kernel.org>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>