GFS2: Add "norecovery" mount option as a synonym for "spectator"

XFS supports the "norecovery" mount option which is basically the
same as the GFS2 spectator mode. This adds support for "norecovery"
as a synonym for spectator mode, which is hopefully a more obvious
description of what it actually does.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>