enable lsof when unmounting nfs clients

Because the lsof interface is strange, use the verbose flag so we get
some output even if there is no use, and ignore errors because it will
return 1 if there's no filesystem use.

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #336 from ceph/wip-nfs

enable lsof when unmounting nfs clients