NFSv4: Fix the sync mount option for nfs4 mounts

The sync mount option stopped working for NFSv4 mounts after commit
c02d7adf (NFSv4: Replace nfs4_path_walk() with
FS path lookup in a private namespace).  If MS_SYNCHRONOUS is set in the
super_block that we're cloning from, then it should be set in the new
super_block as well.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>