Do not flush NFS node from NFS VOP_SET_TEXT().

The more appropriate place to do the flushing is VOP_OPEN(). This was
uncovered because VOP_SET_TEXT() is now called with the vnode'
vm_object rlocked, which is incompatible with the flush operations.

After the move, there is no need for NFS-specific VOP_SET_TEXT
overload.

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MFC after: 30 days