rgmanager: fix nfsrestart option to be effective

The original patch was still racy in some conditions
as other rpc.* and nfs* processes were holding a lock on the filesystem.

stopping nfs in kernel is simply not enough.

this fixed version does stop nfs completely and re-instante nfs exports.

Resolves: rhbz#822053

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>