perf trace: Fixup thread refcounting

perf trace: Fixup thread refcounting

In trace__vfs_getname() and when checking if a thread is filtered in
trace__process_sample() we were not dropping the reference obtained via
machine__findnew_thread(), fix it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-9gc470phavxwxv5d9w7ck8ev@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>