[BUG] RWX filesystem is not frozen for snapshots

[joshimoo]

Since the mount of the longhorn volume happens inside of the share-manager pod and not the host namespace.
The host has no mount point for this volume and therefore it cannot be frozen (i.e. cannot find a mount point to freeze)

We could consider doing the mount in the host namespace then doing a bind mount into the share-manager pod.
This might impact potential failover in the HA scenario where the clients need to reconnect, so that needs to be evaluated at that point in time while working on the RWX HA solution.

fsfreeze context: #2187
NFS HA issue: #2293