tools/bpftool: Add bpftool support for bpf map element iterator

The optional parameter "map MAP" can be added to "bpftool iter"
command to create a bpf iterator for map elements. For example,
  bpftool iter pin ./prog.o /sys/fs/bpf/p1 map id 333

For map element bpf iterator "map MAP" parameter is required.
Otherwise, bpf link creation will return an error.

Quentin Monnet kindly provided bash-completion implementation
for new "map MAP" option.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184119.590799-1-yhs@fb.com

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

@@ -17,14 +17,15 @@ SYNOPSIS
 ITER COMMANDS
 ===================
 
-|	**bpftool** **iter pin** *OBJ* *PATH*
+|	**bpftool** **iter pin** *OBJ* *PATH* [**map** *MAP*]
 |	**bpftool** **iter help**
 |
 |	*OBJ* := /a/file/of/bpf_iter_target.o
+|	*MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
 
 DESCRIPTION
 ===========
-	**bpftool iter pin** *OBJ* *PATH*
+	**bpftool iter pin** *OBJ* *PATH* [**map** *MAP*]
 		  A bpf iterator combines a kernel iterating of
 		  particular kernel data (e.g., tasks, bpf_maps, etc.)
 		  and a bpf program called for each kernel data object
@@ -37,6 +38,12 @@ DESCRIPTION
 		  character ('.'), which is reserved for future extensions
 		  of *bpffs*.
 
+		  Map element bpf iterator requires an additional parameter
+		  *MAP* so bpf program can iterate over map elements for
+		  that map. User can have a bpf program in kernel to run
+		  with each map element, do checking, filtering, aggregation,
+		  etc. without copying data to user space.
+
 		  User can then *cat PATH* to see the bpf iterator output.
 
 	**bpftool iter help**
@@ -64,6 +71,13 @@ EXAMPLES
    Create a file-based bpf iterator from bpf_iter_netlink.o and pin it
    to /sys/fs/bpf/my_netlink
 
+**# bpftool iter pin bpf_iter_hashmap.o /sys/fs/bpf/my_hashmap map id 20**
+
+::
+
+   Create a file-based bpf iterator from bpf_iter_hashmap.o and map with
+   id 20, and pin it to /sys/fs/bpf/my_hashmap
+
 SEE ALSO
 ========
 	**bpf**\ (2),