rcutorture: Fix memory leak in rcu_test_debug_objects()

The kernel memory leak detector located the following:

unreferenced object 0xffff95d941135b50 (size 16):
  comm "swapper/0", pid 1, jiffies 4294667610 (age 1367.451s)
  hex dump (first 16 bytes):
    f0 c6 c2 bd d9 95 ff ff 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000bc81d9b1>] kmem_cache_alloc_trace+0x2f6/0x500
    [<00000000d28be229>] rcu_torture_init+0x1235/0x1354
    [<0000000032c3acd9>] do_one_initcall+0x51/0x210
    [<000000003c117727>] kernel_init_freeable+0x205/0x259
    [<000000003961f965>] kernel_init+0x1a/0x120
    [<000000001998f890>] ret_from_fork+0x22/0x30

This is caused by the rcu_test_debug_objects() function allocating an
rcu_head structure, then failing to free it.  This commit therefore adds
the needed kfree() after the last use of this structure.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>