For some file system types such as ZFS, it is possible to copy/clone the file system so that the fileno for files
do not change, but the fsid for the file system does change.
Since the fsid is used in file handles (mostly used by the NFS server), this will result in a change in the file
handle for a file and ESTALE (stale file handle) errors if the cloned file system is used in a failover NFS server.
This patch adds a generic mount option "fsid=<N>" which replaces the fsid chosen by the file system with "<N>"
at mount time. The option can only be used during initial mounting and not during a "mount -u".