kernel/cgroup.c: kfree(NULL) is legal

Reduces object file size a bit:

Before:
$ size kernel/cgroup.o
   text    data     bss     dec     hex filename
  21593    7804    4924   34321    8611 kernel/cgroup.o
After:
$ size kernel/cgroup.o
   text    data     bss     dec     hex filename
  21537    7744    4924   34205    859d kernel/cgroup.o

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>