Work-around READDIRPLUS problem with .zfs/ and .zfs/snapshot/ directories

Work-around READDIRPLUS problem with .zfs/ and .zfs/snapshot/ directories
by just returning EOPNOTSUPP. This will allow NFS server to fall back to
regular READDIR.

Note that converting inode number to snapshot's vnode is expensive operation.
Snapshots are stored in AVL tree, but based on their names, not inode numbers,
so to convert inode to snapshot vnode we have to interate over all snalshots.

This is not a problem in OpenSolaris, because in their READDIRPLUS
implementation they use VOP_LOOKUP() on d_name, instead of VFS_VGET() on
d_fileno as we do.

PR:		kern/125149
Reported by:	Weldon Godfrey <wgodfrey@ena.com>
Analysis by:	Jaakko Heinonen <jh@saunalahti.fi>
MFC after:	3 days