bpftool: Match maps by name

This patch implements lookup by name for maps and changes the behavior of
lookups by tag to be consistent with prog subcommands.  Similarly to
program subcommands, the show and dump commands will return all maps with
the given name (or tag), whereas other commands will error out if several
maps have the same name (resp. tag).

When a map has BTF info, it is dumped in JSON with available BTF info.
This patch requires that all matched maps have BTF info before switching
the output format to JSON.

Signed-off-by: Paul Chaignon <paul.chaignon@orange.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/8de1c9f273860b3ea1680502928f4da2336b853e.1576263640.git.paul.chaignon@gmail.com

tools/bpf/bpftool/Documentation/bpftool-map.rst

@@ -39,7 +39,7 @@ MAP COMMANDS
 |	**bpftool** **map freeze**     *MAP*
 |	**bpftool** **map help**
 |
-|	*MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
+|	*MAP* := { **id** *MAP_ID* | **pinned** *FILE* | **name** *MAP_NAME* }
 |	*DATA* := { [**hex**] *BYTES* }
 |	*PROG* := { **id** *PROG_ID* | **pinned** *FILE* | **tag** *PROG_TAG* | **name** *PROG_NAME* }
 |	*VALUE* := { *DATA* | *MAP* | *PROG* }
@@ -55,8 +55,9 @@ DESCRIPTION
 ===========
 	**bpftool map { show | list }**   [*MAP*]
 		  Show information about loaded maps.  If *MAP* is specified
-		  show information only about given map, otherwise list all
-		  maps currently loaded on the system.
+		  show information only about given maps, otherwise list all
+		  maps currently loaded on the system.  In case of **name**,
+		  *MAP* may match several maps which will all be shown.
 
 		  Output will start with map ID followed by map type and
 		  zero or more named attributes (depending on kernel version).
@@ -66,7 +67,8 @@ DESCRIPTION
 		  as *FILE*.
 
 	**bpftool map dump**    *MAP*
-		  Dump all entries in a given *MAP*.
+		  Dump all entries in a given *MAP*.  In case of **name**,
+		  *MAP* may match several maps which will all be dumped.
 
 	**bpftool map update**  *MAP* [**key** *DATA*] [**value** *VALUE*] [*UPDATE_FLAGS*]
 		  Update map entry for a given *KEY*.