Support .zfs/snapshot access via NFS

This should fix #616 - support .zfs/snapshot access via NFS.

Current solution works until HAVE_MOUNT_NODEV disabled - more info can be found in :  73d84cf876a883ae42eaaedfb6eeaff62706df88 

During the development few issues not related directly to #616 have bin fixed as well:
* fix invalid fileid for snapshot root dentry : a78466c0e2f6722db1afcd0d87115c432daf94ed
*  set MNT_SHRINKABLE on snapshot mount instead of occasional post mount getattr : 3c4061cd0a3242bab05a50341eb1fd65da487344
* zfsctl: don't sync (to disk) incore only nodes : 2f8344d0aa24933b287732c7d88d3be9e0af1a4c