cgroup: start switching to fs_context

Unfortunately, cgroup is tangled into kernfs infrastructure.
To avoid converting all kernfs-based filesystems at once,
we need to untangle the remount part of things, instead of
having it go through kernfs_sop_remount_fs().  Fortunately,
it's not hard to do.

This commit just gets cgroup/cgroup1 to use fs_context to
deliver options on mount and remount paths.  Parsing those
is going to be done in the next commits; for now we do
pretty much what legacy case does.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>