Fix nfs4 umount to not used umount procedure

This fix umount.nfs4's BUG, patch cleanup some useless code.

1. Combine nfs3_umount() and nfs2_umount() to nfs_umount()
2. If it is a nfs4 remote file system or unkown mount version, do not
   probe remote mount port and not send UMNT request wich is not necessary

Signed-off-by: Wei Yongjun <yjwei@nanjing-fnst.com>
Signed-off-by: Neil Brown <neilb@suse.de>