tools/showstack: add configurable rank parameters

Signed-off-by: pengyiqiang <pengyiqiang@xiaomi.com>

tools/showstack.sh

@@ -29,10 +29,17 @@ function analyse()
 }
 
 if [[ $# == 0 ]]; then
-  echo "usage: $0 <dir>..."
+  echo "usage: $0 <rank> <dir>..."
   exit
 fi
 
+rank=20
+
+if [[ "$1" =~ ^[0-9]+$ ]]; then
+  rank=$1
+  shift
+fi
+
 for dir in $@; do
-  analyse $dir 20
+  analyse $dir $rank
 done