NFS: Don't hard-code the fs_type when submounting

Hard-coding the fstype causes "nfs4" mounts to appear as "nfs",
which breaks scripts that do "umount -at nfs4".

Reported-by: Patrick Steinhardt <ps@pks.im>
Fixes: f2aedb71 ("NFS: Add fs_context support.")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>