cgroup: update and fix parsing of "cgroup.subtree_control"

I was confused that strsep() was equivalent to strtok_r() in skipping
over consecutive delimiters.  strsep() just splits at the first
occurrence of one of the delimiters which makes the parsing very
inflexible, which makes allowing multiple whitespace chars as
delimters kinda moot.  Let's just be consistently strict and require
list of tokens separated by spaces.  This is what
Documentation/cgroups/unified-hierarchy.txt describes too.

Also, parsing may access beyond the end of the string if the string
ends with spaces or is zero-length.  Make sure it skips zero-length
tokens.  Note that this also ensures that the parser doesn't puke on
multiple consecutive spaces.

v2: Add zero-length token skipping.

v3: Added missing space after "==".  Spotted by Li.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>