smbtorture: Add flapping.xfs

We copy a couple of tests to consider flapping from flapping.glusterfs.
These also fail on xfs.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>

testcases/smbtorture/selftest/flapping.xfs

@@ -0,0 +1,5 @@
+# This is a known fail for samba4 env. We explicitly mark env as
+# samba3 which means we don't ever match with this knownfail.
+^samba3.smb2.create.quota-fake-file
+
+^samba3.smb2.session.*