tools lib fd array: Allow associating a pointer cookie with each entry

Add a 'ptr' field to fdarray->priv array.

This feature will be used by following commits, which introduce
muiltiple 'struct perf_mmap' arrays for different types of mapping.

Because of this, during fdarray__filter(), a simple 'idx' is not enough.

Add a pointer cookie that allows to directly associate a 'struct
perf_mmap' pointer to an fdarray entry.

Signed-off-by: Wang Nan <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: He Kuang <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Nilay Vaish <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Zefan Li <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/api/fd/array.h

···
 	struct pollfd *entries;
 	union {
 		int    idx;
+		void   *ptr;
 	} *priv;
 };