Use correct UMNT protocol to do umount

This patch do following things:

1. When unmounting, we should try /proc/mounts first. Only then in
   /etc/mtab if it wasn't found.
2. Reject name passed to us which is not starts with a '/' or not
   contains a ':'
3. Fix the method to get UMNT protocol

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