use getCacheStat before lock, fixes nodejs/node-v0.x-archive#3821

Previous code did not handle the cache dir's permissions consistently.
If the first lock was done as sudo, the cache directory was not user
writable.
Removed the now unnecessary guard and collapsed the then function.

lib/cache.js

@@ -1195,17 +1195,11 @@ function lockFileName (u) {
   return path.resolve(npm.config.get("cache"), h + "-" + c + ".lock")
 }
 
-var madeCache = false
 var myLocks = {}
 function lock (u, cb) {
   // the cache dir needs to exist already for this.
-  if (madeCache) then()
-  else mkdir(npm.config.get("cache"), function (er) {
+  getCacheStat(function (er, cs) {
     if (er) return cb(er)
-    madeCache = true
-    then()
-  })
-  function then () {
     var opts = { stale: npm.config.get("cache-lock-stale")
                , retries: npm.config.get("cache-lock-retries")
                , wait: npm.config.get("cache-lock-wait") }
@@ -1215,7 +1209,7 @@ function lock (u, cb) {
       if (!er) myLocks[lf] = true
       cb(er)
     })
-  }
+  })
 }
 
 function unlock (u, cb) {