cgroup: fix parsing empty mount option string

This fixes the case where all mount options specified are consumed by an
LSM and all that's left is an empty string. In this case cgroupfs should
accept the string and not fail.

How to reproduce (with SELinux enabled):

    # umount /sys/fs/cgroup/unified
    # mount -o context=system_u:object_r:cgroup_t:s0 -t cgroup2 cgroup2 /sys/fs/cgroup/unified
    mount: /sys/fs/cgroup/unified: wrong fs type, bad option, bad superblock on cgroup2, missing codepage or helper program, or other error.
    # dmesg | tail -n 1
    [   31.575952] cgroup: cgroup2: unknown option ""

Fixes: 67e9c74 ("cgroup: replace __DEVEL__sane_behavior with cgroup2 fs type")
[NOTE: should apply on top of commit 5136f63 ("cgroup: implement "nsdelegate" mount option"), older versions need manual rebase]
Suggested-by: Stephen Smalley <[email protected]>
Signed-off-by: Ondrej Mosnacek <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

kernel/cgroup/cgroup.c

@@ -1748,7 +1748,7 @@ static int parse_cgroup_root_flags(char *data, unsigned int *root_flags)
 
 	*root_flags = 0;
 
-	if (!data)
+	if (!data || *data == '\0')
 		return 0;
 
 	while ((token = strsep(&data, ",")) != NULL) {