Fix machine__destroy_kernel_maps() to drop vmlinux_maps references
before filling it with NULL.

Refcnt debugger shows
  ==== [1] ====
  Unreclaimed map: 0x36b1070
  Refcount +1 => 1 at
    ./perf(map__new2+0xb5) [0x4bdec5]
    ./perf(machine__create_kernel_maps+0x72) [0x4bb152]
    ./perf(machine__new_host+0xfa) [0x4bb41a]
    ./perf(init_probe_symbol_maps+0x93) [0x5062d3]
    ./perf() [0x455ffa]
    ./perf(cmd_probe+0x6c) [0x4566bc]
    ./perf() [0x47abc5]
    ./perf(main+0x610) [0x421f90]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f1fc9fc4af5]
    ./perf() [0x4220a9]
  Refcount +1 => 2 at
    ./perf(maps__insert+0x9a) [0x4bfd6a]
    ./perf(machine__create_kernel_maps+0xc3) [0x4bb1a3]
    ./perf(machine__new_host+0xfa) [0x4bb41a]
    ./perf(init_probe_symbol_maps+0x93) [0x5062d3]
    ./perf() [0x455ffa]
    ./perf(cmd_probe+0x6c) [0x4566bc]
    ./perf() [0x47abc5]
    ./perf(main+0x610) [0x421f90]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f1fc9fc4af5]
    ./perf() [0x4220a9]
  Refcount -1 => 1 at
    ./perf(map_groups__exit+0x94) [0x4bea74]
    ./perf(machine__delete+0x3d) [0x4b91fd]
    ./perf(exit_probe_symbol_maps+0x28) [0x506378]
    ./perf() [0x45628a]
    ./perf(cmd_probe+0x6c) [0x4566bc]
    ./perf() [0x47abc5]
    ./perf(main+0x610) [0x421f90]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f1fc9fc4af5]
    ./perf() [0x4220a9]

map__new2() returns map with refcnt = 1, and also map_groups__insert
gets it again in__machine__create_kernel_maps().

machine__destroy_kernel_maps() calls map_groups__remove() to
decrement the refcnt, but before decrement it again (corresponding
to map__new2), it makes vmlinux_maps[type] = NULL. And this may
cause a refcnt leak.
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20151118064022.30709.3897.stgit@localhost.localdomainSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>