tools: bpftool: add a command to display bpftool version

This command can be used to print the version of the tool, which is in
fact the version from Linux taken from usr/include/linux/version.h.

Example usage:

    $ bpftool version
    bpftool v4.14.0

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/tools/bpf/bpftool/Documentation/bpftool.rst b/tools/bpf/bpftool/Documentation/bpftool.rst
index f1df189..45ad8ba 100644
--- a/tools/bpf/bpftool/Documentation/bpftool.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool.rst
@@ -14,6 +14,8 @@
 
 	**bpftool** batch file *FILE*
 
+	**bpftool** version
+
 	*OBJECT* := { **map** | **program** }
 
 	*MAP-COMMANDS* :=