selinux: Allow context mounts for unpriviliged overlayfs

Now overlayfs allow unpriviliged mounts. That is root inside a non-init
user namespace can mount overlayfs. This is being added in 5.11 kernel.

Giuseppe tried to mount overlayfs with option "context" and it failed
with error -EACCESS.

$ su test
$ unshare -rm
$ mkdir -p lower upper work merged
$ mount -t overlay -o lowerdir=lower,workdir=work,upperdir=upper,userxattr,context='system_u:object_r:container_file_t:s0' none merged

This fails with -EACCESS. It works if option "-o context" is not specified.

Little debugging showed that selinux_set_mnt_opts() returns -EACCESS.

So this patch adds "overlay" to the list, where it is fine to specific
context from non init_user_ns.

Reported-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
[PM: trimmed the changelog from the description]
Signed-off-by: Paul Moore <paul@paul-moore.com>