s4-provision: don't test for xattrs if posix:eadb is set

when it is set in smb.conf or on the command line, obey the setting
and don't try to test for system xattr support

source4/scripting/python/samba/ntacls.py

@@ -28,6 +28,7 @@ class XattrBackendError(Exception):
 
 
 def checkset_backend(lp, backend, eadbfile):
+    # if posix:eadb is set, then force the backend
     if backend is not None:
         if backend == "native":
             lp.set("posix:eadb", "")

source4/setup/provision

@@ -218,7 +218,7 @@ elif opts.partitions_only:
 eadb = True
 if opts.use_xattrs == "yes":
 	eadb = False
-elif opts.use_xattrs == "auto" and lp.get("posix:eadb") is None:
+elif opts.use_xattrs == "auto" and not lp.get("posix:eadb"):
 	file = tempfile.NamedTemporaryFile()
 	try:
 		samba.ntacls.setntacl(lp, file.name,