tools: bpftool: optionally show filenames of pinned objects

Making it optional to show file names of pinned objects because
it scans complete bpf-fs filesystem which is costly.
Added option -f|--bpffs. Documentation updated.

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -12,7 +12,7 @@ SYNOPSIS
 
 	**bpftool** [*OPTIONS*] **map** *COMMAND*
 
-	*OPTIONS* := { { **-j** | **--json** } [{ **-p** | **--pretty** }] }
+	*OPTIONS* := { { **-j** | **--json** } [{ **-p** | **--pretty** }] | { **-f** | **--bpffs** } }
 
 	*COMMANDS* :=
 	{ **show** | **dump** | **update** | **lookup** | **getnext** | **delete**
@@ -86,6 +86,9 @@ OPTIONS
 	-p, --pretty
 		  Generate human-readable JSON output. Implies **-j**.
 
+	-f, --bpffs
+		  Show file names of pinned maps.
+
 EXAMPLES
 ========
 **# bpftool map show**