nfsd: Make loop calling VOP_ALLOCATE() iterate until done

The NFSv4.2 Deallocate operation loops on VOP_DEALLOCATE()
while progress is being made (remaining length decreasing).
This patch changes the loop on VOP_ALLOCATE() for the NFSv4.2
Allocate operation do the same, instead of stopping after
an arbitrary 20 iterations.

MFC after: 2 weeks