bpf tools: Add helper function for updating bpf maps elements

Add bpf_map_update_elem() helper function which calls the sys_bpf
syscall to update elements in bpf maps. Upcoming patches will use it to
adjust data in map through the perf command line.

Signed-off-by: He Kuang <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Cc: Zefan Li <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Wang Nan <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/bpf/bpf.c

···
 	log_buf[0] = 0;
 	return sys_bpf(BPF_PROG_LOAD, &attr, sizeof(attr));
 }
+
+int bpf_map_update_elem(int fd, void *key, void *value,
+			u64 flags)
+{
+	union bpf_attr attr;
+
+	bzero(&attr, sizeof(attr));
+	attr.map_fd = fd;
+	attr.key = ptr_to_u64(key);
+	attr.value = ptr_to_u64(value);
+	attr.flags = flags;
+
+	return sys_bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
+}

tools/lib/bpf/bpf.h

···
 		     u32 kern_version, char *log_buf,
 		     size_t log_buf_sz);
 
+int bpf_map_update_elem(int fd, void *key, void *value,
+			u64 flags);
 #endif