perf data: Support single perf.data file directory

Support directory output that contains a regular perf.data file, named
"data". By default the directory is named perf.data i.e.
perf.data
└── data

Most of the infrastructure to support a directory is already there. This
patch makes the changes needed to support the format above.

Presently there is no 'perf record' option to output a directory.

This is preparation for adding support for putting a copy of /proc/kcore in
the directory.

Signed-off-by: Adrian Hunter <[e-mail]>
Reviewed-by: Jiri Olsa <[e-mail]>
Link: http://lore.kernel.org/lkml/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf.data-directory-format.txt

···
+perf.data directory format
+
+DISCLAIMER This is not ABI yet and is subject to possible change
+           in following versions of perf. We will remove this
+           disclaimer once the directory format soaks in.
+
+
+This document describes the on-disk perf.data directory format.
+
+The layout is described by HEADER_DIR_FORMAT feature.
+Currently it holds only version number (0):
+
+  HEADER_DIR_FORMAT = 24
+
+  struct {
+     uint64_t version;
+  }
+
+The current only version value 0 means that:
+  - there is a single perf.data file named 'data' within the directory.
+  e.g.
+
+    $ tree -ps perf.data
+    perf.data
+    └── [-rw-------       25912]  data
+
+Future versions are expected to describe different data files
+layout according to special needs.

tools/perf/builtin-record.c

···
 	size_t padding;
 	u8 pad[8] = {0};
 
-	if (!perf_data__is_pipe(data) && !perf_data__is_dir(data)) {
+	if (!perf_data__is_pipe(data) && perf_data__is_single_file(data)) {
 		off_t file_offset;
 		int fd = perf_data__fd(data);
 		int err;

tools/perf/util/data.c

···
 	DIR *dir;
 	int nr = 0;
 
+	/*
+	 * Directory containing a single regular perf data file which is already
+	 * open, means there is nothing more to do here.
+	 */
+	if (perf_data__is_single_file(data))
+		return 0;
+
 	if (WARN_ON(!data->is_dir))
 		return -EINVAL;
 
···
 	u64 size = data->file.size;
 	int i;
 
-	if (!data->is_dir)
+	if (perf_data__is_single_file(data))
 		return size;
 
 	for (i = 0; i < data->dir.nr; i++) {

tools/perf/util/data.h

···
 };
 
 enum perf_dir_version {
+	PERF_DIR_SINGLE_FILE	= 0,
 	PERF_DIR_VERSION	= 1,
 };
 
···
 static inline bool perf_data__is_dir(struct perf_data *data)
 {
 	return data->is_dir;
+}
+
+static inline bool perf_data__is_single_file(struct perf_data *data)
+{
+	return data->dir.version == PERF_DIR_SINGLE_FILE;
 }
 
 static inline int perf_data__fd(struct perf_data *data)