[sheepdog] [PATCH 1/5] farm: add concurrent access support to the same object

[Christoph Hellwig]

Can you add some comments describing on why we lock here?  The sheepdog
protocol assumes only one VM writes to any given object, so for "normal"
operations it's not nessecary.  The only use case is protecting against
recovery, so is there any reason we can't avoid the locking if a node
is not currently undergoing recovery operations?