ftrace: Fix memory leak in ftrace_graph_release()

ftrace_hash is being kfree'ed in ftrace_graph_release(), however the
->buckets field is not.  This results in a memory leak that is easily
captured by kmemleak:

unreferenced object 0xffff880038afe000 (size 8192):
  comm "trace-cmd", pid 238, jiffies 4294916898 (age 9.736s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff815f561e>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff8113964d>] __kmalloc+0x12d/0x1a0
    [<ffffffff810bf6d1>] alloc_ftrace_hash+0x51/0x80
    [<ffffffff810c0523>] __ftrace_graph_open.isra.39.constprop.46+0xa3/0x100
    [<ffffffff810c05e8>] ftrace_graph_open+0x68/0xa0
    [<ffffffff8114003d>] do_dentry_open.isra.1+0x1bd/0x2d0
    [<ffffffff81140df7>] vfs_open+0x47/0x60
    [<ffffffff81150f95>] path_openat+0x2a5/0x1020
    [<ffffffff81152d6a>] do_filp_open+0x8a/0xf0
    [<ffffffff811411df>] do_sys_...