flapping.cephfs: Add smb2.session tests as knownfail

Many of the tests from smb2.session torture suite are already failing
on XFS for which flapping.xfs already contains an entry to skip any
such failures. Copy over the entry to flapping.cephfs where CephFS is
also not expected to successfully complete the entire test suite.

Signed-off-by: Anoop C S <anoopcs@cryptolab.net>

testcases/smbtorture/selftest/flapping.cephfs

@@ -4,3 +4,5 @@
 # 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.*